# Air Quality Effects on Human Health and Approaches for Its Assessment through Microfluidic Chips
## 3 Air Pollution Sources
### Outdoor Air Pollution
#### 3.2.3 Aerosols and Particulate Matter  
It is estimated that aerosols of anthropogenic origin currently account for approximately 10% of our atmosphere. Non-anthropogenic formed aerosol particles tend to be smaller than particles originating from human activity [79; 99]. Smaller (<1 \(\upmu\)m) and lighter particles tend to stay longer in the air, even for weeks, until they are removed from the atmosphere by precipitation. Larger particles (>10 \(\upmu\)m) will settle to the ground by gravity within hours. Most of the particles that enter the respiratory tract of humans are filtered via cilia and mucus, and removed on the mucociliary escalator [100]. If swallowed, a rather small amount of these particles may be subsequently absorbed through the gastrointestinal tract, while most of them will be excreted [101]. In general, epithelial barriers in the body (skin, lungs, gut, and so on) are very restrictive to nanoparticle entry, as long as they are not compromised by injury or inflammation. Some particles penetrate beyond the larynx, and only very small particles reach and enter the bronchi and lungs. Usually, particles smaller than 10 \(\upmu\)m will reach the bronchioles or alveoli [102]. While the term aerosol encompasses the mixture of solid and liquid matter, PM refers exclusively to solid particles that might be found in the atmosphere [103; 104]. The atmospheric dust dissolved in the atmosphere can carry particles as small as 10 nm, which is in the size range of small viruses. Some examples of where PM can be found in significant amounts are tobacco smoke, soot, or smog. PM can impact the global climate and adversely affect human health. The World Health Organization (WHO) designated airborne PM as a Group 1 carcinogen [105]. Studies showed that there is no safe level of the exposure with PM and with any concentration increase of PM in the air, the cancer rate increases proportionally [106; 107]. The smallest particles with diameters less than 100 nm (nanoparticles or ultrafine particles (UFP)) can further penetrate the alveolar-capillary membrane and migrate into diverse organs, including the brain [108; 109; 110; 111]. Such small particles, also termed diesel particulate matter (DPM), are prevalent in the exhaust from diesel engines, even in the latest and more advanced ones [112; 113]. Emissions from diesel vehicles are thus considered significantly more harmful than those from petrol vehicles [114]. Therefore, it seems recommendable for people that suffer from allergy or asthma to drive a car that consumes petrol rather than diesel (or even better, an electric car). Small sized particles possess a high surface-to-volume ratio, i.e., a given quantity of nano-sized matter has a larger surface area than the same quantity in bulk form. Many carcinogens, like benzopyrenes, can adsorb on the small particles' surface, and are carried through air, eventually reaching the human body. A higher prevalence in the development of cancer was noticed early in towns with highways for diesel-powered trucks when compared to more rural areas [115]. Truck drivers also have higher cancer rates, due to the diesel pollution, when compared to other job categories not directly related to the diesel engine exhaust [116]. Interestingly, the long-haul truck drivers were at lower cancer risk compared to the short-haul truck drivers. The long-haul drivers shut down the windows, which gives an additional protection, as the incoming air is filtered through the air filter. The short-haul truck drivers, in contrast, leave their windows open and are thus exposed to the toxic exhaust originating from their vehicles [117]. The exposure to PM can have a detrimental health effect, and is suspected of harming pregnant women and unborn children. PM can cause oxidative stress, leading to inflammation. Other effects on pregnancy include endocrine disruption and impaired oxygen transport to the placenta, leading to a lower birth weight [118; 119], congenital disabilities (birth defects), premature delivery, or even premature death [120; 121].