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-ductcollapsed-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/