One of the main factors that determine indoor air quality are Chemicals in the Air

Two common causes of complaint regarding poor indoor air quality are bad smells and irritation of the eyes, nose and throat. Such irritation may be induced by specific chemicals, but also by factors such as dry air. Bad smells are not harmful in themselves but may cause unpleasant effects and increase symptoms such as headache, nausea, and irritation of eyes or throat. The hazards associated with chemicals and their effects on health are not always well known, particularly for long term exposures.
​
(GreenFacts- Facts on Health and the Environment)