Volatile Organic Compounds (VOCs) are a large group of liquids and gases, many of which are colorless and odorless. Since they are volatile, the liquids easily vaporize at room temperature.
 
VOCs are of general concern because of their ability to react with other pollutants such as nitrogen oxides in the lower atmosphere to form ozone. High concentrations of ozone at ground level can harm human health, damage crops and affect materials such as rubber.
 
Some VOCs may be directly harmful in form of carcinogens to human health. These can, contribute to global warming via methane gas or harm stratospheric ozone via methyl bromide. The ozone layer is needed to shield the earth’s surface from harmful ultra violet radiation.

UK Environment Agency