Most cities throughout the world have major air-quality issues, which have received more attention in the last decade. Metropolitan population increase, combined with changes in land use as a result of expanding urban areas, is one of the most likely causes of poor air quality. The urban population expansion is produced by (1) migration to cities and (2) an excess of births over deaths in cities, particularly in emerging nations with high birthrates. The intricate combination of natural and artificial environmental conditions in cities results in poor air quality. Air pollution in cities is a major environmental issue, particularly in developing nations. The urban atmosphere's air pollution path consists of the emission and transport of air pollutants, resulting in ambient air pollution. Different elements have an impact on each section of the trip. Motor vehicle emissions are a major source of pollution all around the world. Air contaminants are distributed, diluted, and subjected to photochemical processes during transmission. Most of our industrial and agricultural activities, transportation functions, and energy generation systems produce damaging gaseous emissions. Transboundary issues have been identified in addition to local effects such as olfactory nuisance, health implications, crop damage, and smog production. These include stratospheric ozone depletion, acid rain, and the greenhouse effect. The public's worry over air pollution is expanding significantly. Many countries are implementing environmental protection programs to address these issues. Biofiltration is the oldest biotechnological method for removal of undesired off-gas components. Air pollution can be caused by certain gases in the environment. For example, a gas called ozone is a major source of air pollution in cities. Ozone is a greenhouse gas that has both positive and negative effects on our environment. Everything depends on where you are in the Earth's atmosphere.

 A new era of air pollution research

The period of successful abatement of traditional air pollutants came to an end 20 years ago, with a comprehensive study of the health impacts of ambient particles. At the concentrations seen in the developed world in the late 1970s, serious health impacts were thought to be unlikely. However, air pollution has resurfaced as a major environmental health hazard in the two decades since then. One reason is that, while traditional fossil fuel combustion pollution is now present in considerably lower amounts than it was 50 years ago, other components have become more prominent.

Pollutants of current interest: ozone, particulates, nitrogen dioxide

Now that sulphur dioxide levels have dropped dramatically, the focus has shifted to ozone, nitrogen dioxide, and particles. Before delving deeper into these contaminants, some context is required to place our topic in a broader global context. Indoor pollution from the use of biomass fuels occurs at concentrations that are orders of magnitude higher than those now seen in the developed world for millions of people living in rural areas in developing countries.