“Sick Building Syndrome” (SBS) is used to describe a situation in which the occupants of a building experience acute health or comfort-related effects that seem to be linked directly to the time spent in the building. No specific illness or cause can be identified. However, SBS causes are frequently pinned down to flaws in the heating, ventilation, and air conditioning (HVAC) systems.

Microorganisms, namely bacteria, fungi, and mould can easily penetrate through the filter of air-conditioning systems. The internal environment of air-conditioning systems are humid and become ideal breeding grounds for microorganisms to proliferate. These microorganisms might spread to every corner of the building through the air duct system.

This situation could deteriorate the indoor air quality (IAQ) and pose adverse health effects to occupants in the building such as headache, fatigue, irritation of eyes, nose and/or throat, aggravated asthma, and others.

The World Health Organisation (WHO) reported that 30% of new and remodelled buildings worldwide have poor IAQ. Compromised IAQ could adversely impact human health and subsequently reduce overall productivity. 

(Please check out the following link for more on this topic:  http://www.nbcnews.com/health/health-news/sick-building-fungi-release-toxin-directly-air-study-finds-n776126)