Examples of collapsed air ducts, due to too much weight: Heating duct collapsed on students taking an exam in Kent, UK, May 2009. From http://www.dailymail.co.uk/news/article-1180788/Revealed-The-pictures-inside-exam-hall-heating-duct-collapsed-pupils.html

8 children and 6 adults were injured when this duct collapsed at Shaler Area Elementary School, August 2013: From http://wesa.fm/post/da-zappala-no-criminal-charges-shaler-area-elementary-school-ductwork-collapse

Duct collapse at a car dealership in Boston in October 2014. From http://www.wcvb.com/news/air-duct-collapses-on-workers-cars-at-dealership/29001756

Looking inside an air duct that has a lot of accumulated dust. From http://inspectapedia.com/aircond/Ducts_in_Slabs.php

Monroe Restoration advertisement about cleaning ducts. From http://monroerestoration.com/

Hughes Environmental advertisement about cleaning ducts. From https://hughesenv.com/industrial-duct-cleaning-prevent-dust-explosions/