# Air Quality Effects on Human Health and Approaches for Its Assessment through Microfluidic Chips
## 3 Air Pollution Sources
### Outdoor Air Pollution
#### Human-Made (Anthropogenic) Pollution Sources  
Biological systems were always exposed to non-anthropogenic airborne pollutants throughout evolution, and might thus have developed mechanisms to counteract possible detrimental effects. For example, forest fires and volcanic eruptions are non-anthropogenic sources for particulate matter (PM) in the biosphere, thus entering the respiratory tracts of animals since they evolved [86]. Airborne viruses have challenged the immune systems of living organisms throughout their evolution, a process that is ongoing [87]. With the discovery of fire usage, humans started to burn wood to cook, to keep themselves warm, to keep away predators and more. With the development of agriculture, the land was cleared by igniting forest fires, a practice that is ongoing in the Amazonas region or Indonesia. Therefore, air pollution can be regarded as something that accompanied humans from their earliest history until today. However, the amount and composition of airborne pollutants has changed due to anthropogenic activities, and might thus pose a challenge to human health [88]. In some developing countries, coal combustion is the primary method for heating homes and supplying energy. Coal mining and burning contributed to the famous hazes in England and the USA with the beginning of the industrial revolution, a problem that is still present in some developing countries. Power plants utilizing fossil fuels, such as petroleum (oil) fuel or biomass (dung, wood, and crop waste), also contribute to smoke emission. Industrial processes burning coal or petroleum regularly emit SO\({}_{\text{X}}\) and SO\({}_{2}\), as some coal and petroleum fractions contain sulfur compounds. The released SO\({}_{2}\) can be further oxidized in the air (for example in the presence of NO\({}_{2}\) as a catalyst), thereby forming sulfuric acid (H\({}_{2}\)SO\({}_{4}\)), resulting in acid rain. Most vehicles, including cars, aircraft, and marine vessels, utilize non-renewable energy sources, such as the petroleum products gasoline or diesel [89]. The emissions from motor vehicles are one of the leading contributors to air pollution [90]. Their exhaust contains not only CO\({}_{2}\) and water, but also many other toxic, cancerogenic gases and small particles, capable of penetrating deeply into lungs. Incomplete combustion of fuel or coal produces CO that can be found in the exhaust of vehicles, factories, but also furnaces meant for heating homes. Methane (CH\({}_{4}\)) can be generated from waste deposits in landfills. If methane is not collected, it escapes into the atmosphere. VOCs can be emitted by sewage and garbage, but also by wastewater treatment plants and other industries. Aromatic VOCs, such as toluene and xylene, are known to be carcinogens. Other VOCs are sensed as odors that may severely impair the living comfort of humans. When the two primary pollutants nitric oxide (NO\({}_{\mathrm{A}}\)) and VOCs mix and react in the air, secondary pollutants, so-called strong oxidants like ozone (O\({}_{3}\)) or even the stable oxidant peroxyacetyl nitrate (C\({}_{2}\)H\({}_{3}\)NO\({}_{5}\)), can be formed. As we discussed previously, strong oxidants can damage the mucosa and other human tissues. Another class of pollutants with significant impact are persistent organic pollutants (POPs). POPs are organic compounds that have been and continue to be synthesized in huge amounts, as pharmaceuticals, industrial chemicals, solvents, and pesticides. POPs include compounds such as polychlorinated biphenyls (PCBs), that were used as heat exchange fluids, or dichlorodiphenyltrichloroethane (DDT), which was widely used as insecticide during World War II (WWII). POPs are resistant to environmental degradation through chemical, biological, and photolytic processes, and thus, tend to accumulate in the environment [91; 92]. POPs are typically halogenated organic compounds, and are capable of entering the gas phase at environmental temperatures.