Lesson 3 How does radon enter a home?. Slide 3-1 Predicted average indoor radon levels (U.S.) This...
-
Upload
yadiel-hoyt -
Category
Documents
-
view
218 -
download
3
Transcript of Lesson 3 How does radon enter a home?. Slide 3-1 Predicted average indoor radon levels (U.S.) This...
Lesson 3
How does radon enter a home?
Slide 3-2
Predicted average indoor radon levels (U.S.)
This map cannot be used to determine the radon level of a particular home. Homes with high levels of radon have been found in all three zones.
All homes—regardless of their zone—should be tested.
Zone 1 (red): Zone 1 (red): greater than greater than
4 pCi/L 4 pCi/L Zone 2 (orange): Zone 2 (orange):
2-4 pCi/L2-4 pCi/LZone 3 (yellow): Zone 3 (yellow):
less than less than 2 pCi/L2 pCi/L
Slide 3-3
Predicted average indoor radon levels (Connecticut)
Zone 1 (red): Zone 1 (red): greater than greater than
4 pCi/L 4 pCi/L Zone 2 (orange): Zone 2 (orange):
2-4 pCi/L2-4 pCi/LZone 3 (yellow): Zone 3 (yellow):
less than less than 2 pCi/L2 pCi/L
This map cannot be used to determine the radon level of a particular home. Homes with high levels of radon have been found in all three zones.
All homes—regardless of their zone—should be tested.
Slide 3-4
Requirements for radon to enter a home
1. A source of radon2. A mechanism to transport
radon from the source into the home
3. An opening or pathway into the home
Slide 3-5
Level of radon in a home
Depends on• Strength of radon sources: most
important factor• Distance of the sources from the
home• Ease of transport into the home• Ventilation in the home• Environmental factors
Slide 3-6
Where radon comes from
Uranium-238
Thorium-234
Thorium-230
Radium-226
Radon-222
Protactinium-234See handout 3-1
Slide 3-7
Sources of radon
• Soil and rock: most common source• Groundwater• Building materials containing
uranium and radium
Slide 3-8
Radon source Soil and rock
• Uranium is present at about 0.5 to 5 parts per million (ppm) in common rocks and soil– Uranium and radium especially
common in granites, shales, and limestones
– Under a home, they can be source of radon
• Uranium decays into radium– Radium decays into radon
Slide 3-9
Overview Radon from soil and rock
– Drains– Sump holes– Dirt floors– Construction joints– Spaces around service pipes
• Radon enters soil gas• Soil gas moves from
ground into air in home• Usually enters through
foundation– Cracks in walls and
floors
Slide 3-10
Radon source Groundwater
• Where underlying rocks contain high levels of uranium
• Where homes rely on groundwater from private wells or small public waterworks as the main water source
Usually a problem only in small, closed water systems
Slide 3-11
Radon in small, closed groundwater systems
• Radon does not have time to decay into harmless by-products before entering a home
• Once inside the home, radon escapes from the water into the air during normal household activities:– Showering– Washing clothes or dishes– Flushing toilets
Slide 3-12
Radon in other water
• Use surface water (lakes, streams, rivers, and reservoirs)– Radon usually escapes into
air before it reaches a home
• Use groundwater from large public systems– Water is aerated (mixed with air) and escapes– Longer transit times allow most of remaining
radon to decay into harmless products
Usually not a problem where homes
Slide 3-13
Radon sourceBuilding materials
• Materials sometimes contain radium or uranium– Brick– Granite – Concrete products– Sheet rock– Materials contaminated with radioactive refuse
(rarely used)• Usually contribute little to indoor radon
Slide 3-14
Strength of radon sources
Soil: moderate to weak radon source
Rock: strong radon
sourceRock: strong radon source
Rock:moderate
radon source
Even homes next to each other may have different radon sources, with different strengths.
Groundwater containing radon
Slide 3-15
Questions?
• About radon sources
• Next– Mechanisms that transport radon into a
home
Slide 3-16
Radon transport mechanisms
Average contribution to radon in a home
Diffusion(1-4%)
Emanation (2-5%)
Outgassing (less than
1%)
Air pressure differences (85-90% )
Push or pull radon into a home
Slide 3-17
Main radon transport mechanism Air pressure differences
• Home creates small vacuum (negative air pressure) – Draws in soil gas,
including radon
• Vacuum caused by– Temperature differences
between outside and inside air (stack effect)
– Mechanical systems– Environmental factors
Slide 3-18
Air pressure differencesStack effect
• Heated indoor air rises and escapes through cracks and holes at top of home– Creates positive air
pressure at top of home– Creates negative air
pressure (vacuum) at bottom
• Vacuum draws in soil gas, including radon
• Effect is greatest during coldest months
• Thermal bypasses increase effect
Positive pressure
Warm air
Negative pressure
Soil gas (including radon)
Slide 3-19
Air pressure differences Mechanical systems
• Heating, ventilation, and air conditioning (HVAC) systems– Air distribution
blowers– Furnaces– Boilers– Wood-burning
fireplaces– Woodstoves– Other combustion
systems
• Home exhaust systems (vent air to outside)– Clothes dryers– Exhaust fans in
bathrooms, kitchens, or attics
– Central vacuum cleaners
Slide 3-20
Air pressure differences Environmental effects
• Weather– Seasons– Rain, snow,
and frost– Wind
• Other factors that seal the soil around a home
Slide 3-21
Environmental effectsSeasons
Warm weather• Open windows• Equal pressure
indoors and outdoors– L ess radon
enters
• Good ventilation dilutes radon concentration
Cold weather • Closed windows• Lower pressure
indoors– More radon enters
• Poor ventilation traps radon inside
Usually more radon enters in winter
Slide 3-22
Complex effects of good ventilation (open windows)
• Reduces vacuum effect– Generally reduces radon
entry– Dilutes radon in home
• But may also increase stack effect– Thereby increases radon
entry
When you measure radon in short-term tests, should windows be
open or closed?
Slide 3-23
Complex effects of good ventilation (open windows)
When you measure radon in short-term
tests, windows must be kept
closed
• Reduces vacuum effect– Generally reduces radon
entry– Dilutes radon in home
• But may also increase stack effect– Thereby increases radon
entry
Slide 3-24
Environmental effects Rain, snow, and frost
• Rain, snow, and frost can seal the soil– Prevent radon from
escaping from around the foundation
• Rain can force soil gas into the home
Slide 3-25
Wind
Lower pressure
down-wind
Higher pressure upwind
Don’t measure radon during high winds because results may not show typical levels.
Downwind draft effect • Changes pressure around the home• Higher pressure in soil as wind pushes
beneath soil
Environmental effectsWind
Slide 3-26
Environmental effectsFactors that seal the soil
• Asphalt or concrete driveways
• Concrete patios
Prevent radon from escaping into outdoor air
Slide 3-27
Summary Air pressure
• Main mechanism that brings radon into a home– Difference in air pressure between indoor and
outdoor air
• Main causes of air pressure differences– Temperature differences between indoor and
outdoor air– Mechanical systems– Environmental factors
Slide 3-28
Other radon transport mechanisms
Diffusion
• Radon concentration is higher at its source (underlying soil or foundation) than in indoor air
• Radon moves from area of higher concentration to area of lower concentration
Higher radon concentration
Lower radonconcentration
Diffusion = movement through materials
Slide 3-29
Radon
Other radon transport mechanisms Emanation
• Some rocks and other building materials contain uranium or radium
Radon↑
Radium↑
Uranium
Emanation = emission of gas from a surface by radioactive decay
• As these elements decay, radon may be created on their surfaces
• The radon may be emitted into a room
• Emanation rate depends on– Amount of radioactive materials– Surface area of the materials
Slide 3-30
Other radon transport mechanisms Outgassing
Outgassing = release of radon gas from water
Slide 3-31
Questions?
• About mechanisms that transport radon into a home
• Next– Pathways that allow radon to enter a
home
Slide 3-32
Radon pathways into the home
• Natural – Pores or empty spaces in soil– Cracks in underlying rocks– Earthen areas in basements
• Artificial– Fill below foundations– Trenches for utility lines and plumbing– Water drainage systems– Other openings in foundations
Slide 3-33
Pathway characteristics
• Ease with which air moves through the pathway
• Distance from radium (radon source)• Connections with other pathways
Slide 3-34
ReviewRequirements for radon entry
1. Radon source2. Mechanism to transport radon from
source into home3. Pathways into the home
These factors determine the amount of radon that enters a home.
Slide 3-35
Radon levels vary in space
• In space– From geographic area to geographic
area– From home to home– From level to level within a home
• Usually highest in lower levels of home• Higher readings in upper levels suggest
unusual radon entry factors
Slide 3-36
Radon levels vary over time
• In time– From season to
season – From day to day– From hour to hour
• With changes to the home (such as additions)
Because radon levels vary, testing must be done over a period of time
• Factors – Air pressure changes– Wind speed and
direction– Indoor and outdoor
temperature changes– Rain, snow, and frost– Use of mechanical
exhaust systems
Slide 3-37
Why this informationmatters
• Air pressure differences• Ventilation• Environmental effects
Procedures for measuring radon are designed to control for these factors:
Slide 3-38
Summary
• Every home should be tested for radon
• Requirements for radon to enter a home– Radon source– Mechanism to transport radon– Pathway
See handout 3-3
Slide 3-39
Summary
• Sources– Soil and rock– Groundwater– Building materials
• Mechanisms– Air pressure
differences– Diffusion– Emanation– Outgassing
• Pathways– Distance from radon
source– Ease with which air moves– Connections with other
pathways
• Variation in radon levels– In space– In time– With changes in the home
Slide 3-40
SummaryHow radon enters a home
Slide 3-41
Activity
• Importance of testing every home
• Factors that determine the radon level in a home
• Most common sources of radon
• Common mechanisms of radon entry
• Pathways into the home• Variations in radon levels
over space and time
Hypothetical client: an attorney
For a hypothetical client, summarize how radon enters a home
Slide 3-42
Activity review
• Is the explanation complete?• Is the explanation clear?• Does the explanation contain the
right level of technical detail for this client?
See handout 3-2 for a sample explanation
Slide 3-43
Questions
• About how radon enters a home
Slide 3-44
Check your understanding
• See handout 3-4