Selecting Respirators for Control of Worker Exposure to Infectious Aerosols • Author(s): Nicole Vars McCullough , PhD; Lisa M. Brosseau , ScD, CIHSource: Infection Control and Hospital Epidemiology, Vol. 20, No. 2 (February 1999), pp. 136-144Published by: The University of Chicago Press on behalf of The Society for Healthcare Epidemiologyof AmericaStable URL: http://www.jstor.org/stable/10.1086/501602 .

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136 INFECTION CONTROL AND HOSPITAL EPIDEMIOLOGY February 1999

Selecting Respirators for Control of Worker Exposureto Infectious Aerosols

Nicole Vars McCullough, PhD; Lisa M. Brosseau, ScD, CIH

A method for selecting respirators for protection in infec-tious aerosol environments was developed, building on a proce-dure used to choose respiratory protection for environments con-taining nonbiological contaminants. Modifications in the tradition-al respirator selection method are proposed for situations whereinformation on occupational exposure limits, toxicity, and airborneconcentrations is absent. Toxicity is determined from risk rank-ings proposed by a variety of organizations. The nature of the activ-ity allows assessment of source generation, which is combined

with room volume and airflow to obtain a ranking of airborne con-centration. Finally, concentration and toxicity ranks determine aminimum assigned protection factor, which corresponds to a res-pirator class. Case studies are presented to illustrate the proposeddecision logic. For each situation, the procedure yielded choicesthat were both protective and reasonable. These results suggestthat the procedure will be applicable to a variety of settings for arange of infectious organisms (Infect Control Hosp Epidemiol1999;20:136-144).

For many years, respiratory protection has beenworn in industrial settings to minimize exposures to a vari-ety of nonbiological aerosols. Recently, respirator use hasexpanded; respirators now are being worn to lower biolog-ical aerosol exposures in both industrial and nonindustrialsettings. In some instances, they replace surgical masks,which originally were designed for the purpose of minimiz-ing patient exposure to materials expelled by the wearer.When aerosols created by the patient or a medical proce-dure present a hazard to a medical worker, respirators arethe only appropriate means of protection. No matter howwell the filter media collects particles, surgical masks can-not be considered respirators, because they are notdesigned to fit the wearer’s face. In addition, they are notcertified by the National Institute for Occupational Safetyand Health (NIOSH) and do not meet the OccupationalSafety and Health Administration (OSHA)’s regulatory cri-teria as respiratory protective devices.1,2

Respiratory protection has been recommendedspecifically by the Centers for Disease Control andPrevention (CDC) and NIOSH for lowering occupationalexposure to tuberculosis.3,4 OSHA currently is enforcingthe use of NIOSH-certified respirators in five workplaces:healthcare settings, correctional institutions, homelessshelters, long-term–care facilities for the elderly, and drug

treatment facilities.5 Respirators also are appropriate for avariety of other occupational environments where expo-sure to airborne infectious organisms is likely.6-10

The currently accepted selection procedures11,12 can-not be used to choose respirators for exposures tobioaerosols. Measurement of airborne concentrations isnot presently feasible; consequently, there are no occupa-tional exposure limits for biological organisms. A strategyfor selecting respirators for protection from infectiousorganisms is proposed here, using qualitative estimates ofairborne concentration and toxicity. This procedure isintended for use by practitioners in the fields of infectioncontrol and occupational health and will be applicable to avariety of infectious-aerosol environments.

BACKGROUND

Respirators differ in the protection they offer,depending on how well they fit a wearer. A respirator’s fitfactor (FF) is determined by the ratio of the ambient con-centration outside (Co) the respirator to the concentrationinside (Ci) the facepiece:

(1) FF=Co/Ci

An assigned protection factor (APF) is used to

From the Division of Environmental and Occupational Health, School of Public Health, University of Minnesota, Minneapolis, Minnesota.Address reprint requests to Lisa M. Brosseau, ScD, CIH, University of Minnesota, School of Public Health, Division of Environmental and

Occupational Health, 420 Delaware St SE, Box 807 Mayo, Minneapolis, MN 55455.97-OA-125. McCullough NV, Brosseau LM. Selecting respirators for control of worker exposure to infectious aerosols. Infect Control Hosp

Epidemiol 1999;20:136-144.

ABSTRACT

Readers’ Forum

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Vol. 20 No. 2 READERS’ FORUM 137

describe the average protection offered by a class of respi-rators when properly fitted and worn by most workers. Theappropriate APF is determined for each exposure by thefollowing:

(2) APF=Co/OEL

where OEL is the occupational exposure limit for the cont-aminant of interest.

The standard procedure for selecting respiratorsinvolves a series of decisions that systematically eliminateclasses of respirators until one is found that meets all of thecriteria of the worker and workplace.11,12 The APF is calcu-lated using equation 2, and a respirator class with an equalor greater APF is selected.

Selecting respirators is an important component of arespiratory protection program, as mandated by OSHA.7 Atypical respirator decision procedure involves the followingsteps:

1. Determine whether there is adequate oxygen inthe environment. If not, a supplied air respirator is the onlyappropriate choice.

2. Determine if peak or emergency concentrations(often referred to as “immediately dangerous to life orhealth” [IDLH]) could cause immediate incapacitation ofthe respirator wearer and prevent escape from the environ-ment. In these situations, only supplied air respirators witha backup air supply or self-contained breathing apparatusare appropriate.

3. Measure or estimate the airborne contaminantconcentration (Co).

4. Determine the appropriate OEL for the contami-nant. The OEL and Co are used to calculate the minimumAPF (equation 2), and a table is consulted for the appropri-ate respirator class. Assigned protection factors recom-mended by NIOSH and the American National StandardsInstitute (ANSI) are shown in Table 111,13; the differencesoriginate from their interpretations of experimental results.

5. If an air-purifying respirator is selected, the physi-cal nature of the contaminant(s) then guides the choice ofair-cleaning element. Aerosols require a filter, whereaschemical cartridges are employed for gases and vapors.Combinations of these are available for mixed exposures.

6. Finally, consider any special circumstances thatmight require upgrading the class of respirator. For exam-ple, if splashes are likely, a full-facepiece respirator shouldbe selected. Issues such as physiologic load, comfort, wear-ing time, battery life, and work practices also should beconsidered when selecting a respirator for a particularworker and workplace.12

The CDC did not employ such a selection processwhen recommending respirators for exposures toMycobacterium tuberculosis. The guidelines state only themaximum allowable face-seal leakage (10%, equivalent toan APF of 10) and minimum allowable filter performance(collection efficiency of 95% for 1 µm particles).3 In a worst-case scenario, these guidelines permit exposures inside therespirator (Ci) of up to 15% of the ambient concentration,

irrespective of its level. When the Co of infectious particlesis high, a respirator wearer may not be protected ade-quately by these recommendations. This is particularlyimportant for exposures to M tuberculosis, because thisorganism has an infectious dose of less than 10.14

One investigator has developed a model for estimat-ing exposure by predicting the probability of infection15:

(3) Probability{Infection}=1�e�(I q b t f / QR

)

where I is the number of tuberculosis patients in the room,q is the number of “quanta” (disease-causing units) emittedper hour per tuberculosis patient, b is the worker’s volu-metric breathing rate, t is the worker’s cumulative timespent in the room, f is the fraction of inhaled quanta thatdeposit in the alveolar region, and QR is the supply airflowinto the room. While these variables are important in deter-mining the probability of infection, some such as q and fmay not be available, and others such as b may be difficultto assess for each situation. This model has served as thebasis of a new standard proposed by an ANSI committee forrespiratory protection in infectious aerosol environments(which is presently in review).16

While the CDC’s recommendations are simplisticand may be unprotective, the method described above iscomplicated and infeasible for many situations. The selec-tion process presented here represents a compromisebetween the two.

Respirator selection using the current process fornonbiological aerosol exposures requires informationabout all of the following:

● contaminant air concentration, ● contaminant toxicity, ● OEL, ● particle-size distribution, and● characteristics of use.

TABLE 1ASSIGNED PROTECTION FACTORS FOR RESPIRATORY PROTECTIVE

DEVICES

Respirator Type ANSI13 NIOSH11

Single-use or disposable dust mist 10 5Air-purifying (half mask) 10 10Air-purifying (full facepiece) 100 10/50*PAPR (hood or helmet) 1,000* 25PAPR (loose-fitting facepiece) 25 25PAPR (half mask) 50 50*PAPR (full facepiece, tight-fitting) 100/1,000* 50*Supplied air (full facepiece, continuous flow) 1,000 50Supplied air (full facepiece, pressure demand) 1,000 2,000SCBA, (full facepiece, pressure demand) <10,000 10,000

Abbreviations: ANSI, American National Standards Institute; NIOSH, National Institute forOccupational Safety and Health; PAPR, powered air-purifying respirator; SCBA, self-containedbreathing apparatus.* When equipped with high-efficiency particulate air filters.

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138 INFECTION CONTROL AND HOSPITAL EPIDEMIOLOGY February 1999

However, much of this information is missing forinfectious aerosols. Currently, there are no relevant mea-surements of airborne infectious organism concentrations.Without these, the relationship between exposure andhealth effects cannot be assessed easily. Consequently, noexposure limits have been developed for these contami-nants. In the following section, we describe some methodsfor qualitative rankings of concentration and toxicity thatcan be used in place of steps 3 and 4 of the decision processoutlined above.

To select an appropriate level of respiratory protec-tion for exposures to infectious organisms, it is necessaryto assess the specific circumstances in which exposureoccurs. The methods described below allow decisions thatuse existing estimates of risk while considering the condi-tions of a particular situation. Conditions consideredinclude those that are common to all inhaled hazards (eg,aerosol generation rate, degree and quality of ventilation,spatial dimensions) and those particular to microorgan-isms (eg, pathogenicity and infectivity).

Air ConcentrationWhen local exhaust ventilation (eg, fume hoods) is

lacking, as is often the case in healthcare settings, generalexhaust ventilation is relied upon for contaminant removal.The effectiveness of general ventilation depends on airflow

patterns near the source and how effectively clean air ismixed with contaminated air. When clean supply air enter-ing a space is mixed instantaneously with contaminated air,the latter is diluted at a constant rate over time, resulting inperfect mixing.17 Predictive models of contaminant levelsbased on ventilation rates assume that the air within a par-ticular space will be mixed perfectly. In most cases, howev-er, such an assumption is unrealistic; therefore, safety fac-tors are included in these models to account for nonperfectmixing.

Several methods have been developed for estimatingair concentrations of contaminants as a function of ventila-tion and time.17,18 While these yield good estimates of con-taminant concentration, they rely on quantitative measure-ments of the rate at which a contaminant is generated. Forexample, one model employs a simplistic relationshipbetween concentration (C, particles/cm3), ventilation volu-metric flow (Q, cm3/min), and generation rate (G, parti-cles/min):

(4) C=(G/Q)

This model has been adapted here by deriving quali-tative ranks for generation rate (Grank) and ventilation flow(Qrank) to arrive at a ranking for concentration (Crank):

TABLE 2RECOMMENDED VENTILATION RATES FOR HEALTHCARE AND PUBLIC FACILITIES20,21

Minimum ACHHealthcare Facilities Outdoor Air Total Recirculation Allowed

Patient room 2 4 OptionalIsolation room* 2 6 NoPatient corridor 2 4 OptionalExamination room 2 6 OptionalTrauma room 5 12 NoAutopsy room 2 12 NoOperating room (all outdoor-air system) 15 15 NoOperating room (recirculating-air system) 5 25 YesDelivery room (all outdoor-air system) 15 15 NoDelivery room (recirculating-air system) 5 25 Yes

Public Facilities ACH Minimum Outdoor Air (L/s/Person)

Restaurants 8-12 2.5Bars 15-20 5Airport terminals 8-12 2.5Bus terminals 8-12 2.5Classrooms NA 8Commercial aircraft NA 7Pools 4-6 †

Abbreviations: ACH, air changes per hour; NA, not available.* The Centers for Disease Control and Prevention has recommended new facilities be constructed using 12 ACH.3† The method for calculating minimum air requirements depends on a number of variables.21

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Vol. 20 No. 2 READERS’ FORUM 139

(5) Crank=(Grank/Qrank)

Flow can be expressed either in air changes per hour(ACH) or in cubic feet per minute (ft3/min); the conversionbetween these is:

(6) Q(ft3/min)=(V(ft3))(ACH)(1 hr/60 min).

Typical ventilation rates for healthcare and publicfacilities are shown in Table 2, and suggested flow rankings(Qrank) are given in Table 3.19-21

The ranges for Qrank shown in Table 3 were derivedby applying equation 6 to the range of ACH suggested bythe American Society of Heating, Refrigerating, and Air-Conditioning Engineers for different hospital areas.20 Ifaverage room dimensions of 10 ft�10 ft�10 ft areassumed (giving a room volume of 1,000 ft3), ventilationflow (Q) ranged from 30 ft3/min for clean linen and equip-ment storage areas to 400 ft3/min for surgical suites.Ranges of flow were selected to give ranks for Q of 1 to 5(Table 3).

To account for imperfect mixing, a safety factor oftenis included in equation 4. However, selecting an appropriatevalue for this safety factor can be difficult, because littleinformation is available about the parameters influencingmixing in a particular space. We recommend a differentapproach. For each situation, the effectiveness of airflow

should be considered and Qrank lowered accordingly. Forexample, if a worker is located near the source of an air-borne infectious aerosol (as occurs during bronchoscopyor autopsy), airflow in the room probably will be ineffec-tive at lowering this worker’s exposure. The value of Qrankfor this worker should be lower than for a worker locatedat some distance from the source. As another example,the movement of air through a room may be compro-mised by obstacles or blocked vents. In this case, air mov-ing through the room may not dilute all spaces equally,suggesting a lower value for Qrank. The opinion of a facili-ties engineer or ventilation expert may be appropriate forsuch situations.

Generation rate (G) is a function of the amount(mass or number) of aerosol generated per unit time andrepresents the contribution of a particular source to theairborne contaminant concentration. There are no directmeasurements of infectious-aerosol generation rates fromhumans. Based on observed rates of infection followingexposure to an individual infected with tuberculosis, oneinvestigator estimated the generation rate at approximate-ly 250 infectious units per hour and the airborne concen-tration (resulting from expellations) to be approximately 1infectious unit per 2,000 L of air.22

We suggest a method that qualitatively ranks gen-eration rate based on the source type or activity (Table 4).For example, a person with infectious tuberculosis may be

TABLE 3RANKING VENTILATION FLOW (Q

RANK) USING THE AMERICAN SOCIETY OF HEATING, REFRIGERATING, AND AIR-CONDITIONING ENGINEERS

STANDARDS FOR HOSPITAL AREAS

CalculatedFlow Using Room Range

Minimum Volume of 1,000 ft3 of Flow RankingExample Locations Total ACH20 (ft3/min) (ft3/min) of Flow (Qrank)

Surgery 25 417 >300 5Autopsy, nursery suite, isolation room 12 200 151-300 4Examination room, laboratory, intensive 6 100 75-150 3

care, recovery roomSterile storage, patient room 4 67 10-75 2Clean linen storage, equipment storage 2 33 <10 1

Abbreviation: ACH, air changes per hour.

TABLE 4 RANKING GENERATION RATE (G

RANK) BY SOURCE AND ACTIVITY

Ranking ofHumans as Source of GenerationInfectious Aerosols* Activities as Source of Infectious Aerosols Rate (Grank)

Not talking, coughing, or sneezing Manipulation with no possibility of aerosol generation (ie, slide preparation) 1Coughing and sneezing with mouth covered Manipulation with low risk of accidental aerosol generation (ie, culture propagation) 2Coughing and sneezing with mouth uncovered Manipulation with high risk of accidental aerosol generation (ie, centrifugation) 3Respiratory therapy, autopsy, or dissection Deliberate aerosolization (ie, aerosol research) 4

* Applicable to persons with suspected or confirmed infectious disease transmitted by the airborne route.

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140 INFECTION CONTROL AND HOSPITAL EPIDEMIOLOGY February 1999

considered a source whose rate of particle generation willvary with activity. Investigators have demonstrated thatinfectious particles emitted in coughs, sneezes, andspeaking can contain infectious M tuberculosis.23 Thus, asource who is sleeping and not coughing or sneezing mayexpel a smaller number of infectious particles in compari-son to a source expelling larger numbers during respira-tory therapy.

Laboratory and technical procedures also may gen-erate aerosols. Activities that can create airborne particlesinclude bursting bubbles, breaking liquid films, mixing,aerating, spilling or splashing liquids, ultrasonic vibration,or leaking fluid under pressure. Equipment that may createaerosols include mixers, shakers, blenders, ultrasoniccleaners, centrifuges, syringes, needles, and pipettes.24 TheCDC addresses the creation of aerosols in its laboratorybiosafety levels. Activities for which there is no risk of acci-dental aerosol creation (eg, cell staining) may be per-formed at biosafety level 2 when using organisms that maycause disease through inhalation. On the other hand,biosafety level 3 is required for the same organisms whenpropagation and manipulation of cultures occurs or equip-ment that can generate aerosols is used.25

Estimation of Contaminant HazardEventual manifestation of disease depends on organ-

ism characteristics (eg, pathogenicity, virulence),24 envi-ronmental factors (eg, desiccation), host factors (eg, sus-ceptibility), exposure, and dose. These factors, alone or incombination, can be used to rank the risk of exposure to aparticular organism.

Infectious dose (the number of organisms necessaryfor infection to develop) has been used by others as an indi-cator of infectious-organism toxicity.15,26 For example, adose of less than 10 infectious units (ie, cells) of M tuber-culosis, Coxiella burnetti, or Francisella tularensis has beenshown to cause infection in 25% to 50% of the popula-tion.14,27,28 However, infectious dose is not available for allorganisms or diseases. Therefore, other measures of dis-ease severity, such as case fatality rates or published riskrankings, may be necessary for determining the level ofprotection needed in a given situation.

Case fatality rate is defined as the percentage ofinfected cases that die. This statistic may indicate the sever-ity of the disease or effectiveness of current treatments. Asan example, measles generally has a low fatality rate,whereas Ebola and clinically evident invasive aspergillosisdemonstrate very high case fatality rates. However, report-ed case fatality rates for a particular organism or diseasemay differ greatly by region and population characteristics(immune status or treatment availability). The case fatalityrate of many diseases is much higher for those who do notreceive treatment or who are immune compromised. Casefatality rate may be a useful measure of toxicity or risk,however, if data are available for circumstances similar tothose of interest.

Rather than relying on infectious dose or case fatali-ty rate, we recommend using the risk-based rankings

developed or proposed by a variety of organizations (Table5), including the CDC, National Institutes of Health (NIH),European Federation of Biotechnology, EuropeanCommission, Canadian Laboratory Centre for DiseaseControl, and the World Health Organization.28 The defini-tions for these risk rankings, which range from 1 (no risk)to 4 (high risk), vary somewhat among the organizations.One should review an organization’s definitions beforeselecting a specific risk ranking. As an example, the NIHemploys the following definitions for risk groups 1through 4:

● Risk group 1: agents not associated with diseasein healthy adult humans.

● Risk group 2: agents that are associated withhuman disease that is rarely serious and for which preven-tive or therapeutic interventions are often available.

● Risk group 3: agents that are associated withserious or lethal human disease for which preventive ortherapeutic interventions may be available (high individualrisk but low community risk).

● Risk group 4: agents that are likely to cause seriousor lethal human disease for which preventive or therapeuticinterventions are not usually available (high individual riskand high community risk).

The ranking scheme developed by the CDC classi-fies risk based on both organism and activity. Biosafety lev-els take into consideration an agents’ virulence, patho-genicity, biological stability, route of spread, and communi-cability; the nature or function of the laboratory; the proce-dures and manipulations involving the agent; the endemic-ity of the agent; and the availability of effective vaccines ortherapeutic measures.25

Although these ranking systems are convenient,they have two important drawbacks: (1) the classificationsdo not always agree for a particular organism, and (2) infec-tion is typically the only end point considered, althoughmany organisms also are capable of causing serious healtheffects through immunologic response or intoxication.28

Differences in classification are mainly the result of region-al differences in treatment availability, climactic distribu-tion of organisms, and degree of endemicity. The WorldHealth Organization recommends that each region developits own ranking based on mode of transmission, host range,and availability of preventive measures and treatmentbefore or following exposure.

Particle SizeFilters collect particles differentially, depending on

particle size, characteristics of the filter material, and air-flow through the filter.34 The size of airborne infectiousorganisms has been the subject of considerable discus-sion and speculation. In its guidelines on tuberculosis, theCDC relied on work from the 1950s that found that air-borne bacteria were present on particles, called “dropletnuclei,” ranging in size from 1 to 5 µm.35 However, theanalytic methods employed in this research were unableto identify particles less than 1 µm. Predictive equationsand direct measurements of airborne bacteria suggest

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Vol. 20 No. 2 READERS’ FORUM 141

that the aerodynamic diameter of a single bacterial parti-cle could be 0.3 µm or smaller,34 although bacteria may beas large as 30 µm.35

Filters display a range of efficiency, which for a givenflow depends largely on the particle aerodynamic diame-ter.34 Because the less efficient respirator filters (dust-mistand dust-fume-mist) certified under older NIOSH tests (30CFR 11) display considerable variability in collection of par-ticles less than 1 µm, only high-efficiency particulate air(HEPA) filters will collect all infectious bacterial organismswith greater than 99% efficiency.36 On the other hand, mostinfectious organisms should be collected with high effi-ciency by respirator filters certified with the new NIOSHtests (42 CFR 84).1 In countries where respirator filters arecertified by agencies other than NIOSH, the efficiency inthe size range of infectious particles should be consideredwhen selecting a respirator.

Modified Selection Procedure for Infectious BacterialAerosols

Using the qualitative rankings described above, arevised respirator decision procedure would consist of thefollowing steps (since steps 1 and 2 will not change, the listbegins with step 3):

3. Estimate the airborne concentration. This isaccomplished by determining the ranks of G and Q (Tables 3 and 4). As described above, Qrank should beadjusted, if necessary, for the effectiveness with which airis carried away from the generating source. The ranking ofC (Crank) is then calculated using equation 5, and rangesfrom 0.2 to 4.

4. Determine an appropriate OEL. As described above,an estimate of toxicity can be used as a surrogate for the OEL(Table 5); if conflicting ranks occur, choose the most conser-vative (highest). Finally, select a minimum APF (Figure),

TABLE 5ESTIMATES OF TOXICITY FROM RISK RANKINGS FOR ORGANISMS COMMONLY USED IN LABORATORIES AND BIOTECHNOLOGY

Organism EFB 1989* CDC 1974† CDC 1993‡ NIH 1994§ LCDC 1996�� EC 1998¶

BacteriaMycobacterium tuberculosis 3 3 2,3# 3 3 3Bacillus anthracis 3 2 2,3# 2 3 3Bordetella pertussis 2 2 2,3# 2 2 2Brucella 3 3 2,3# 3 3 3Chlamydia psittaci (avian strains) 3 3 2,3# 2 3 3Coxiella burnetii 3 3 2,3# 3 3 3Francisella tularensis (type A) 3 NA 2,3# 3 3 3F tularensis (type B) NA NA 2,3# 3 2 2Legionella 2 NA 2,3# 2 2 2Yersinia pestis 3 3 2,3# 3 3 3

VirusesEbola virus 4 4 4 4 4 4Hantavirus (Sin Nombre) NA NA 2,3# 3 3 3Influenza virus 2 NA 2 2 2 2Marburg virus 4 4 4 4 4 4Measles virus 2 2 NA 2 2 2Mumps virus 2 NA NA 2 2 2Rubella virus 2 NA NA 2 2 2Varicella virus 2 NA 2 2 2 2

FungiAspergillus fumigatus 2 NA NA NA 2 2Blastomyces dermatitidis 3 NA 2 2 3 3Coccidioides immitis 3 NA 2,3# 3 3 3Histoplasma capsulatum 3 NA 2,3# 3 3 3Penicillium marneffei NA NA NA 2 NA 3

ParasitesToxoplasma gondii 3 NA 2 2 2 2

Abbreviation: NA, not available or not ranked by this organization.* European Federation of Biotechnology, proposed.29

† Centers for Disease Control and Prevention.30

‡ Centers for Disease Control and Prevention.25

§ National Institutes of Health.31

�� Laboratory Centre for Disease Control.32

¶ European Community.33

# Level 2 if no manipulation of culture; any manipulations require level-3 practices.

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142 INFECTION CONTROL AND HOSPITAL EPIDEMIOLOGY February 1999

using the ranks of C and T. The values in the Figure wereassigned using the CDC’s recommendations for respiratoruse in healthcare settings as a benchmark (described in thefirst example below).

5. Determine the physical nature of the contaminantin order to select the appropriate air-cleaning element. Inaddition to selecting the appropriate filtration for exposuresto airborne infectious organisms, a gas and vapor cartridgemay be necessary for exposure to volatile organic com-pounds, such as those produced by some molds and bacte-ria. In addition, many organisms can infect eyes andmucous membranes, requiring the eye protection offeredby a full facepiece. If other materials are present (eg, otheraerosols, gases, or vapors), these also must be consideredwhen selecting the appropriate air-cleaning elements.

6. Finally, consider the special circumstances sur-rounding the use of this respirator in a particular environ-ment. In addition to the issues mentioned previously, circum-stances unique to infectious aerosols may pertain. For exam-ple, increases in organism resistance or host susceptibilitymay suggest higher levels of protection. Contact with filterscarries some potential for re-aerosolization or skin contami-nation, increasing the risk of infection.37-39 If frequentlyreused during the day or from day to day, respirators withenclosed filter media will be more appropriate than the dis-posable type with unprotected filters. If communication withpatients is necessary, a respirator with a speaking diaphragmmay be useful.

EXAMPLES

Five examples are described. These were selected toillustrate a range of exposure scenarios with several differ-ent organisms.

Example 1The first example was chosen to represent the type

of situation described by the CDC in its 1994 tuberculosis

guidelines.3 This example was used as a benchmark for cre-ating the Figure. In this scenario, a healthcare worker iscaring for a patient with pulmonary tuberculosis housed ina typical patient room.

This example begins with step 3, ruling out fire, oxy-gen deficiency, and IDLH conditions. The contaminant ofconcern is an infectious aerosol expelled by the patient.The lowest value of G is assigned (Grank=1); it is assumedthat the patient is not coughing. An airflow (Q) of 27ft3/min is determined by assuming a typical patient roomsize (400 ft3) and four ACH, which gives a Qrank of 2. Forthis example, there is no reason to adjust the value of Qrank.Using equation 5, Crank=Grank/Qrank=1/2=0.5. M tuberculosiscarries a Trank of 3 (Table 5); an APF of 10 is found for thesevalues of Crank and Trank (Figure).

A healthcare worker having direct contact with thepatient will need to communicate verbally with thepatient and other workers, and may enter and exit thearea several times throughout a shift. Using the APFsdeveloped by ANSI (Table 1), a half-facepiece air-purifying respirator can be used. In accordance with theCDC guidelines for tuberculosis, HEPA filters or any fil-ter certified under the new NIOSH procedures are appro-priate for this exposure.1

Respiratory therapy often is carried out in the sameroom for a patient with infectious tuberculosis. If thiswere true for this example, the rank for G would increaseto 4. Using equation 5, Crank then becomes 4/2=2. Usingthe same value of 3 for Trank, the APF for this situation isnow 25. Employees working with this patient during res-piratory therapy activities should wear a powered air-purifying respirator (PAPR) with a loose-fitting facepiece(using ANSI’s numbers in Table 1) and equipped withHEPA filters.

Example 2Workers performing autopsies are at considerable

risk of contracting infectious diseases, particularly tuber-culosis.40,41 This example describes a process for selectingrespiratory protection for such a situation. Again, it isassumed that fire will not be encountered, oxygen is ade-quate, concentrations are below IDLH levels, and infectiousaerosols are the contaminant of primary concern.

A room volume of 1,000 ft3 and 12 air changes perhour (Table 2) were selected. These give a value for Q of 200ft3/min, which corresponds to a Qrank of 4 (Table 3). Therank of G will be 4 (Table 4). Professional judgment suggeststhat general room ventilation may not effectively lower work-er exposures during autopsies, thus Qrank should be loweredby at least one rank. In this case, we suggest lowering Qrankby two ranks to a value of 2, because workers are locatednear the point of aerosol generation. If Qrank=2, thenCrank=4/2=2. M tuberculosis is assigned a toxicity ranking(Trank) of 3 (Table 5). The Figure shows an APF of 25 forthese values of Crank and Trank. Using ANSI’s numbers, anAPF of 25 leads to selection of a PAPR with a loose-fittingfacepiece. HEPA filters or any filter certified under the newNIOSH procedures may be employed.1

FIGURE. Identifying assigned protection factors using toxicity and concentra-tion ranks.

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Vol. 20 No. 2 READERS’ FORUM 143

Example 3Whirlpool spas have been implicated in the transmis-

sion of Legionella pneumophila resulting in clinical cases oflegionnaires’ disease.19,42 This example will describe theselection of respiratory protection for workers involved incleaning and disinfecting a contaminated spa. It isassumed that the jets will not be operated during the pro-cedure and that the whirlpool is operated in a 1,500 ft3

room with 4 ACH. It also is assumed that fire will not beencountered, there is adequate oxygen, concentrationsare below IDLH levels, and infectious aerosols are thecontaminant of concern.

Using this information, Q=100 ft3/min, which givesit a rank of 3. It may be necessary to lower Qrank, depend-ing on the effectiveness of room ventilation in adequatelyclearing out airborne organisms. Because workers arenear the point of generation and we assume the whirlpoolhad been stopped only recently, we lowered Qrank to 2 toaccount for generally ineffective ventilation. The rankingof generation rate would be 2 with the jets turned off;however, because the jets had been operated in the recentpast (resulting in the initial cases) and the organisms arepotentially small enough to remain airborne, the genera-tion ranking is increased to 3. Using equation 5,Crank=3/2=1.5. A toxicity ranking (Trank) of 2 is assigned tolegionnaires’ disease. From the Figure, a Crank of 1.5 andTrank of 2 result in an APF of 10. This leads to the selectionof a half-facepiece PAPR, which may be single-use, dis-posable, or reuseable (using ANSI numbers from Table1). HEPA filters or any filter certified under the newNIOSH certification standard will be appropriate.1 TheWisconsin Department of Health has recommended theuse of half- or full-facepiece respirators equipped withHEPA filters when cleaning cooling towers and relatedequipment.43

Example 4An official from the Department of Health has been

asked to investigate a Hantavirus outbreak and mustenter the house where the infected individual may havebeen exposed to Hantavirus pulmonary syndrome virus.To determine respiratory protection for this official, it isassumed that the internal house volume is approximately7,000 ft3 and that ventilation is low (1 ACH). This gives avalue for Q of 117 ft3/min and a value of 3 for Qrank. Thegeneration rate is probably low, as most particles willbecome airborne through human or air movement; avalue of 2 is given to Grank (if activities such as cleaninghave created more disturbance of rodent droppings, theranking for G should be increased appropriately). Usingequation 5, Crank=2/3=0.7 (round up to 1). Hantavirus car-ries a toxicity ranking (Trank) of 3. These values of Crankand Trank indicate that a respirator with an APF of 10should be worn. A disposable HEPA filter or reuseablePAPR with replaceable high-efficiency filters will beappropriate for minimizing this official’s exposure toHantavirus during the inspection. The CDC recommendsthe use of a half-facepiece respirator or powered air-puri-

fying respirator with HEPA filters when cleaning homesof people infected with Hantavirus or working in buildingswith heavy rodent infestation.9

Example 5A field researcher plans to enter a cave where

Histoplasma capsulatum fungi may be present in the batdroppings lining the cave floor. Assuming low or no venti-lation (Qrank=1) and a value of 2 for the ranking of genera-tion rate (due to movement of researcher and bats throughthe cave), Crank=2/1=2. This fungus carries a toxicity rank-ing of 3; these values of Crank and Trank indicate that a respi-rator with an APF of 25 should be worn. A PAPR with high-efficiency filters will give the protection necessary (ANSInumbers from Table 1). HEPA filters are necessary,because viruses are very small. In this exposure, addition-al consideration should be given to the level of oxygen inthe cave and whether wear time suggests a more comfort-able respirator. The CDC recommends using a half-facepiece respirator when surveying areas where birdsroost.44

DISCUSSION

Qualitative risk assessment such as this must beused with care. Underlying assumptions and the degree ofconjecture should be understood clearly before applyingthis model to real-life situations. We have attempted to illus-trate the hypothetical nature of this procedure by present-ing a variety of methods for ranking generation rate andtoxicity. The adjustment of Q for effectiveness requires pro-fessional judgment, presumably by someone with appropri-ate training (such as an industrial hygienist).

It is important to keep in mind that the Figure is justone model among many describing the relationshipbetween C and T. With the exception of the benchmarkcase described above, no validation of this relationship cur-rently is possible. Although the outcomes of each of theexamples appear reasonable, one could easily argue thatthey result in either not enough or too much protection.Only when more is known about the effect of respiratorwear on lowering the risk of infectious disease can bettermodels be developed.

As illustrated in the five examples, assumptions canmake all the difference. Therefore, it is important that allassumptions be stated clearly. For the more toxic infectiousaerosols, such as M tuberculosis, it is best to select the mostconservative rankings for C and T to ensure adequateworker protection.

Despite these shortcomings, we believe this proce-dure represents a small step toward a more informed res-pirator selection procedure for infectious aerosols. Foreach situation, the procedure yielded choices that appearedto be both protective and reasonable. The examplesdemonstrate that the proposed method is applicable to avariety of settings for a range of infectious organisms.REFERENCES

1. Approval of Respiratory Protective Devices. Code of Federal RegulationsTitle 42, Part 84, 1996:528-593. Washington, DC: Office of the Federal

This content downloaded from 195.78.108.111 on Wed, 21 May 2014 07:42:04 AMAll use subject to JSTOR Terms and Conditions

144 INFECTION CONTROL AND HOSPITAL EPIDEMIOLOGY February 1999

Register, National Archives and Records Administration.2. Respiratory Protection. Code of Federal Regulations Title 29, Part,

1910.134, 1997:413-417. Washington, DC: Office of the Federal Register,National Archives and Records Administration.

3. Centers for Disease Control and Prevention. Guidelines for preventingthe transmission of Mycobacterium tuberculosis in health-care facilities.MMWR 1994;43:1-132.

4. National Institute for Occupational Safety and Health. NIOSHRecommended Guidelines for Personal Respiratory Protection of Workersin Health-Care Facilities Potentially Exposed to Tuberculosis.Morgantown, WV: US Department of Health and Human Services,Public Health Service, Centers for Disease Control, National Institutefor Occupational Safety and Health; 1992.

5. Clark RA. OSHA enforcement policy and procedures for occupationalexposure to tuberculosis. Infect Control Hosp Epidemiol 1993;14:694-699.

6. Centers for Disease Control and Prevention. Legionnaires’ disease asso-ciated with a whirlpool spa display—Virginia, September-October, 1996.MMWR 1997;46:83-86.

7. Christiani DC, Wegman DH. Respiratory disorders. In: Levy BS,Wegman DH, eds. Occupational Health: Recognizing and PreventingWork-Related Disease. 3rd ed. New York, NY: Little, Brown and Co; 1995.

8. Centers for Disease Control and Prevention. Compendium of Psittacosischlamydiosis control. MMWR 1997;46(RR-13):1-13.

9. Centers for Disease Control and Prevention. Hantavirus infection—southwestern United States: interim recommendations for risk reduc-tion. MMWR 1993;42(RR-11):1-12.

10. New York City Department of Health. Guidelines on assessment andremediation of Stachybotrys atra in indoor environments. New York, NY:New York City Human Resources Administration; 1993.

11. Bollinger NJ, Schutz RH. NIOSH Guide to Industrial RespiratoryProtection. Publication no. NIOSH 87-116. Washington, DC: USDepartment of Health and Human Services; 1987.

12. Colton CE, Birkner LR, Brosseau LM, eds. Respiratory Protection: AManual and Guideline. 2nd ed. Akron, OH: American IndustrialHygiene Association; 1991.

13. American National Standards Institute. American National Standard forRespiratory Protection. ANSI Z88.2-1992. New York, NY: ANSI; 1992.

14. Bloom BR, Murray CJL. Tuberculosis: commentary on a reemergentkiller. Science 1992;257:1055-1964.

15. Nicas M. Respiratory protection and the risk of Mycobacterium tubercu-losis infection. Am J Ind Med 1995;27:317-333.

16. Heinsohn P, Birkner J. Status of the ANSI Z88.12 standard: practices forrespiratory protection against infectious aerosols. Kansas City, MO:American Industrial Hygiene Conference and Exhibition; May 1995.Presentation.

17. Marshall JW, Vincent JH, Kuehn TH, Brosseau LM. Studies of ventila-tion efficiency in a protective isolation room by the use of a scale model.Infect Control Hosp Epidemiol 1996;17:5-10.

18. Burgess WA, Ellenbecker MJ, Treitman RD. Ventilation for Control of theWork Environment. New York, NY: John Wiley & Sons; 1989.

19. Jernigan DB, Hofmann J, Cetron MS, Genese CA, Nuorti JP, Fields BS,et al. Outbreak of legionnaires’ disease among cruise ship passengersexposed to a contaminated whirlpool spa. Lancet 1996;347:494-499.

20. American Society of Heating, Refrigerating and Air-ConditioningEngineers. Health facilities. In: 1991 Application Handbook. Atlanta, GA:ASHRAE; 1991:7.5.

21. American Society of Heating, Refrigerating and Air-ConditioningEngineers. Commercial and public buildings. In: 1991 ApplicationHandbook. Atlanta, GA: ASHRAE; 1991;3.2-3.3.

22. Catanzaro A. Nosocomial tuberculosis. Am Rev Respir Dis 1982;125:559-562.

23. Riley RL, O’Grady F. Airborne Infection: Transmission and Control. NewYork, NY: The Macmillan Company; 1961.

24. Lelieveld HLM, Bachmayer H, Boon B, Brunius G, Burki KG, Chmiel A,et al. Safe biotechnology, part 6: safety assessment, in respect of humanhealth, of microorganisms used in biotechnology. Applied Microbiology

and Biotechnology 1995;43:389-393.25. Centers for Disease Control and Prevention. Biosafety in Microbiological

and Biomedical Laboratories. Publication CDC 93-8395. 3rd ed.Washington, DC: Department of Health and Human Services; 1993.

26. Brachman PS. Epidemiology of nosocomial infections. In: Bennet JV,Brachmann PS, eds. Hospital Infections. Boston, MA: Little, Brown andCo; 1979.

27. Burke DS. Immunization against tularemia: analysis of the effectivenessof live Franciscella tularensis vaccine in prevention of laboratory-acquired tularemia. J Infect Dis 1997;135:55-60.

28. Lelieveld HLM, Boon B, Bennett A, Brunius G, Cantley M, Chmiel A,et al. Safe biotechnology, part 7: classification of microorganisms onthe basis of hazard. Applied Microbiology and Biotechnology1996;45:723-729.

29. Frommer WA, Archer L, Brunius G, Collins CH, Donikian R, Frontali C,et al. Safety precautions for handling microorganisms of different riskclasses. Applied Microbiology and Biotechnology 1989;45:723-729.

30. Centers for Disease Control. Classification of Etiologic Agents on theBasis of Hazard. 4th ed. Atlanta, GA: Department of Health and HumanServices, Public Health Service, Centers for Disease Control, Office ofBiosafety; 1974.

31. National Institutes of Health. Appendix B. Classification of human etio-logic agents on the basis of hazard. In: Guidelines for Research InvolvingRecombinant DNA Molecules. Bethesda, MD: Department of Health andHuman Services, Public Health Service, National Institutes of Health;1994. (http://www.nih.gov/od/orda/apndxb.htm)

32. Laboratory Centre for Disease Control. Laboratory Biosafety Guidelines.2nd ed. Kennedy ME, ed. Ottawa, Ontario, Canada: Health Canada,Minister of National Health and Welfare; 1996. (http://www.hc-sc.gc.ca/main/lcdc/web/bmb/biosafty/lbg4K_e.html)

33. Health and Safety Commission, Advisory Committee on DangerousPathogens. Approved List of Biological Agents. 3rd ed. Supplement to:Categorization of Biological Agents According to Hazard and Categories ofContainment. 4th ed. Suffolk, United Kingdom: Health and SafetyExecutive Books; 1998. (http://www.open.gov.uk/hse/agents.htm)

34. Hinds WC. Aerosol Technology. New York, NY: John Wiley and Sons; 1982.35. Wells WF. Airborne Contagion and Air Hygiene. Cambridge, MA:

Harvard University Press; 1955:13-19.36. Respiratory Protective Devices; Tests for Permissibility; Fees. Code of

Federal Regulations, Title 30, Part 11, 1993: 47-111. Washington, DC:Office of the Federal Register, National Archives and RecordsAdministration.

37. Templeton GL, Illing LA, Young L, Cave D, Stead WW, Bates JH. Therisk for transmission of Mycobacterium tuberculosis at the bedside andduring autopsy. Ann Intern Med 1995;122:922-925.

38. Pasanen A-L, Keinanen J, Kalliokoski P, Martikainen PI, Ruuskanen J.Microbial growth on respirator filters from improper storage. Scand JWork Environ Health 1993;19:421-425.

39. Qian Y, Willeke K, Ulvicius V, Grinshpun SA. Particle reentrainmentfrom fibrous filters. Aerosol Science and Technology 1997;27:394-404.

40. Brosseau LM, McCullough NV, Vesley D. Mycobacterial aerosol collec-tion efficiency by respirator and surgical mask filters under varying con-ditions of flow and relative humidity. Applied Occupational andEnvironmental Health 1997;12:435-445.

41. Lundgren R, Norrman E, Asberg I. Tuberculosis infection transmitted atautopsy. Tubercle 1987;68:147-150.

42. Breiman RF, Cozen W, Fields BS, Mastro TD, Carr SJ, Spika JS, et al.Role of air sampling in investigation of an outbreak of legionnaires’ dis-ease associated with exposure to aerosols from an evaporative con-denser. J Infect Dis 1990;161:1257-1261.

43. Wisconsin Department of Health and Social Services. Control ofLegionella in Cooling Towers: Summary Guidelines. Madison, WI:Wisconsin Department of Health; 1987.

44. Centers for Disease Control and Prevention. Histoplasmosis: ProtectingWorkers at Risk. DHHS (NIOSH) publication no. 97-146. Washington,DC: National Institute for Occupational Safety and Health; 1997.

This content downloaded from 195.78.108.111 on Wed, 21 May 2014 07:42:04 AMAll use subject to JSTOR Terms and Conditions