# Air Quality Effects on Human Health and Approaches for Its Assessment through Microfluidic Chips
## 3 Air Pollution Sources
### Outdoor Air Pollution
#### 3.2.1 Natural (Non-Anthropogenic) Pollution Sources  
As mentioned earlier, many natural sources are polluting the air. Radon gas escapes from the Earth's ground and can accumulate in confined areas. In some places on Earth, such as Siberia, methane (CH\({}_{4}\)) is released from the permafrost as a consequence of the temperature rise induced by global warming. Also, large herds of animals, such as cattle, may release considerable amounts of methane. Naturally occurring forest fires release carbon dioxide and carbon monoxide. CO\({}_{2}\) is a naturally occurring gas in the environment, necessary for the growth of plants [72]. These wildlife fires also cause smoke and haze. Especially lightning-triggered fires are common in the forests of northern Canada in summer. Volcanic activity produces not only ash particles, but also sulfur, sulfur dioxide (SO\({}_{X}\), SO\({}_{2}\)), chlorine, and other toxic gases in high quantities. In some regions, the vegetation can be responsible for seasonal haze. Plants release VOCs, which react with SO\({}_{2}\), NOx, and organic carbon compounds, also contributing to the formation of haze [73]. Especially oak, willow, poplar, and black gum trees are known to produce large amounts of VOCs [74].  
Interestingly, the salt spray over the oceans is the strongest contributor to the aerosols and particulates found in Earth's atmosphere, and thus, the main source of non-anthropogenic generated aerosols. [75; 76]. Sea spray consists of sodium chloride (NaCl), with some magnesium (Mg\({}^{2+}\)), sulfate (SO\({}_{4}\)\({}^{2-}\)), calcium (Ca\({}^{2+}\)), potassium (K\({}^{-}\)), and other components present in ocean water. Reports explain how the sea salt in the air removes the pollutants in the atmosphere via cloud forming processes [77]. On the other hand, there are reports on the possibility of secondary product formation from sea salts. The escaped chlorine from sea salt aerosol particles can promote HCl formation, leading to the formation of secondary pollutants, such as ozone or nitryl chloride (CINO\({}_{2}\)) [78].  
Dust storms (or in the context of the desert, sandstorms) are one of the naturally occurring pollution sources, with serious impact on air quality, and thus, human health, especially in Africa and the Middle East [76]. The blow of strong winds can detach and release mineral particles and other sand or soil particles from the Earth's crust, forming aerosols over large areas of land [79]. The main terrestrial sources of airborne dust are the dry lands around North Africa and the Arabian Peninsula [80]. The dust storms are also formed in various Asian countries, including Iran, India, Pakistan, and China [81]. These fine dust particles are blown over long distances, even hundreds and thousands of kilometers. The high-speed surface winds disturb the ground in the deserts of Mongolia, northern China, and Kazakhstan, carrying dust particles over China, and even as far as Korea and Japan. The so-called yellow dust storm (also known as Asian dust storm) is prevalent during late winter and springtime in these regions [82]. It is possible to predict and follow sand storms. One of the best measures to prevent exposure to pollutants from dust storms is to stay indoors. Nevertheless, after sandstorms, many patients will visit doctors at the hospital for pharyngeal diseases, such as allergies and asthma [83]. Dust particles are known as potential carriers of viruses, bacteria, and fungi [84]. Together with soil particles, other toxic pollutants may be carried over by dust storms too. These pollutants include herbicides, pesticides, antibiotics, heavy metals (arsenic (As), lead (Pb), cadmium (Cd), mercury (Hg), chromium (Cr), zinc (Zn), copper (Cu)), and others [85].