Plants like mint, marigolds, pine, rosemary, etc have lovely strong aromas which are due to Volatile Organic Compounds (VOCs) released when you crush their leaves. I cut rosemary and put them in a vase to give my home a good smell naturally. However, too much of the aroma could cause problems for someone with asthma or lung disease.
You can distill the VOCs and they can cause problems. In cotton mills, a problem millworkers dealt with was brown lung which was caused by the inhalation of small cotton fibers. Who would think cotton which we use as a beauty product and the manufacture of quality clothing could be dangerous. Small cotton fibers are not VOC's.
Volatile Organic Compounds are called volatile because they have a high vapor pressure which means they can exist as a gas at normal temperatures. They are organic in that they originate from living organisms.
Large quantities are toxic. Some VOCs like benzene and fossil fuels such as propane are toxic. They have a low solubility in drinking water and can contaminate ground and surface water. A popular remedy for fire ants is to use gasoline. Gasoline is a VOC which can contaminate large quantities of ground water. So it may kill a fire ant nest but it's after effects on the environment is much more serious and cannot be remedied easily.
One problem of VOC's is that they may be released from common building products such as carpet, air fresheners, or photocopiers. A concentration of VOC's in a poorly ventilated building may cause "sick building syndrome" in which people who live or work there have chronic illnesses as a result.