Harmful effects of Hydrocarbons
Hydrocarbons in air by themselves alone cause no harmful effects. However, they undergo chemical reactions in the presence of sunlight and nitrogen oxides. They form photochemical oxidants leading to photochemical smog. This causes irritation in the eyes and lungs leading to respiratory diseases.
Other Pollutants
A number of other pollutants may enter into the atmosphere from many sources. For example, lead comes from automobile exhaust, though this has reduced considerably after the compulsory change over to the use of unleaded fuel by many countries.
Some aldehydes such as acrolin, formaldehyde, aromatic aldehydes have been detected in some areas with concentrations that may be toxic. They are produced due to incomplete combustion of hydrocarbons or by interaction of hydrocarbons with NO2 under the influence of sunlight. Many aldehydes are the precursors of peroxyacyl nitrates (PAN) and peroxy benzoyl nitrates (PBN) which act as photochemical oxidants. Chlorofluorocarbons (CFC), which are widely used as propellants and refrigerants escape into atmosphere. When they reach the upper sphere of the atmosphere they disturb the ozone layer. Other toxic substances include cadmium from paint and plastic production, polychlorinated biphenyls (PCB's) from electrical transformers, and the banned DDT and dioxin ("agent orange") from pesticides.