# Air Quality Effects on Human Health and Approaches for Its Assessment through Microfluidic Chips
## 3 Air Pollution Sources
### Indoor Air Pollution Origins and Prevention
#### 3.1.4 Biological Pollution Sources  
Indoor pollutants originating from biological sources play a significant role in our environmental health. Microorganisms can be pathogenic or non-pathogenic, depending on their ability to challenge or evade the host immune system. Further, the relationship between host and microorganism shifts from mutual to unilateral, in the case of pathogenicity. A vast number of non-pathogenic microorganisms are regularly residing on and inside our body (also called microbiota), where both the microorganisms and our body benefit from such a relationship. However, immune-comprised individuals, or persons with an immunodeficiency, are especially susceptible to infections, as pathogenic bacteria gain access to the body's interior. Furthermore, microorganisms that are considered non-pathogenic might turn pathogenic when the host immune system is not able to prevent their overgrowth. From the \(\approx\)1400 known pathogenic species in total, around one hundred are pathogenic bacteria capable of causing infectious diseases in healthy humans [65]. Since microorganisms can be propagated through air, they are classified and treated as air pollutants that exhibit a hazard and have exposure thresholds. Not only plant pollen or animal dander can be the cause of asthma and allergies, but also microorganisms, like certain bacteria and molds. Improper ventilation and moisture accumulation are suitable conditions for bacterial and mold growth. Especially mold is associated with increased (>50%) humidity levels. Within only two days, mildew can propagate and release spores into the environment. The spores can cause allergic reactions and asthma, even in healthy persons. To avoid the growth and propagation of molds like _Aspergillus_, it is important to dry clothes after washing in an appropriate time frame. The dangers of mold produced mycotoxins are comprehensively reviewed elsewhere [66]. However, the spores themselves do not carry mycotoxins; the more dangerous part is the spore cell wall, that can cause immunoreactions upon exposure. This is especially the case for persons already suffering from asthma or allergies. The pollutants of microorganisms are not bound to surfaces alone (such as to the ceiling, or walls), they can also be distributed throughout the air by attaching to dust and other particles. That is why it is important to keep the indoor environment free from dust. Overgrowth of bacteria due to unclean conditions (such as standing water) can give rise to odor, and in the case of overgrowth of pathogens, may cause various diseases, including lung infection. Some important bacteria in the context of indoor pollution are _Staphylococcus aureus_, _Streptococcus pneumoniae_, and _Mycobacterium tuberculosis_[66]. Of course, a number of other non-pathogenic bacteria populate the human skin, and are an essential part of the gastrointestinal tract. It is known that households remove CO\({}_{2}\), and in turn, release oxygen and water. There have been trials to use indoor plants to reduce some VOC compounds, such as toluene, benzene, and xylene [67]. Especially in the context of extended space travel, the use of plants is of high interest, and research in that area has already been initiated [68]. These studies came to inconclusive results, probably because the uptake of VOCs by plants is very slow, and only insignificant amounts were taken up [69; 70]. It was shown that removal through passive and active ventilation in houses would remove pollutants faster and more efficiently than through plants. Despite these findings, some plants are recommended for removing biological and chemical pollutants, namely, the English ivy, Boston fern, and aloe vera. The plants probably remove the VOCs through absorption on their leaf surfaces. On the other side, there are recommendations not to keep plants that generate high moisture indoors, due to their support to increase humidity levels, which would result in mold growth and subsequently cause respiratory diseases [71].