# Air Quality Effects on Human Health and Approaches for Its Assessment through Microfluidic Chips
## 3 Air Pollution Sources
### Indoor Air Pollution Origins and Prevention
#### 3.1.1. Gaseous Pollutants  
Carbon monoxide (CO) and dioxide (CO\({}_{2}\)) can originate from tobacco smoke or the combustion of fossil fuels. CO is formed in the case of incomplete combustion. Raised levels of CO and CO\({}_{2}\) can deplete oxygen levels in the air, leading to nausea, drowwiness, headaches, unconsciousness, and eventually death. Another source for increased CO\({}_{2}\) levels is human metabolic activity, especially with many occupants enclosed in one room [33]. The control of CO and CO\({}_{2}\) concentrations can be facilitated by implementing a smoke-free environment and plenty of outdoor air ventilation--either by opening the windows or adjusting ventilation levels [34; 35]. Ventilation systems with integrated CO\({}_{2}\) sensors can adjust the ventilation intensity according to the actual CO\({}_{2}\) levels.  
Atmospheric O\({}_{3}\) is formed from free oxygen radicals [36]. O\({}_{3}\) is produced by the sun's ultraviolet light, lightning, air purifiers with ionizing function, and some devices designed to kill bacteria and sterilize [37]. Also, high voltage devices, such as laser printers, lasers, arc welders, and electric motor sparking can generate significant levels of ozone. Usually, the highest concentrations of ozone can be found in the so-called ozone layer that is situated on the top of the stratosphere, around 20 to 30 km above the Earth. Ozone is a very powerful oxidant, damaging the mucous and respiratory tissues in humans and animals. Additionally, some chemicals used for cosmetics (such as skin oil) or cleaning (based on terpene or citrus extracts) may react with ozone, and eventually produce toxic byproducts or even dangerous nanoparticles. Ventilation might fail in reducing indoor ozone levels, as the atmospheric air may be a major source of ozone [38]. A potential solution would be to install ozone filters in ventilation systems. Some larger airplanes have ozone filters to prevent the introduction of ozone from the outside [39].  
Radon (Rn) is a common gaseous pollutant, and a pervasive and serious hazard in an indoor environment. The two most common radioactive elements on Earth are thorium (Th) and uranium (U). Radon is the by-product of the radioactive decay of Th and U to the intermediate product, radium (Ra). Radon is present in various rocks, such as uranium ores, shales, granite, and even limestone, and accumulates in high quantities in mines [40; 41]. Some hot springs and spring waters may contain a high concentration of radon too [42]. Usually, radon enters a building through fundaments, or originates from the building material itself (such as granite). Radon tends to accumulate at the ground level, as it is a heavy gas. Radon mitigation (reducing radon levels) is done by sealing the basement, thus filling any cracks present, and of course, by enough ventilation, possibly with positive pressurization [43; 44]. The half-life of radon is only 3.8 days. Therefore, removing or isolating the source will greatly reduce the hazard within a few weeks. There are kits and even digital radon detectors and various sensors available. Radium and its decay product, \({}^{222}\)Rn, are classified as carcinogens to humans [45]. When gaseous, radon is even more dangerous, as it can be inhaled directly into the lungs. Lung cancer is the main illness caused by radon exposure, even for non-smokers [46; 47; 48; 49; 50]. Some studies point to a synergetic effect of smoking and radon exposure [50]. Furthermore, there are some hints, also inconsistent, in the published literature on the correlation between radon exposure and chronic lymphocytic leukemia [51].