The overall indoor air quality (IAQ) benefits of routine duct cleaning on heating, ventilating, and air conditioning (HVAC) systems continue to be reviewed and researched. Despite more than two decades of research, there is still not enough evidence to draw solid conclusions about duct cleanings benefits on IAQ, occupant health, HVAC system performance, or energy savings, according to a 2010 review of scientific studies on duct cleaning. The review did find clear evidence that ductwork can be contaminated with dust and can act as a reservoir for microbial growth under normal operating conditions. Yet, even when duct cleaning was extremely efficient at removing contaminants within ducts, the effectiveness of reducing indoor air pollutants was highly variable, and, in many cases, post-cleaning levels of contaminants were higher than pre-cleaning levels.