The planet Earth is the only known and available biosphere for human beings. Therefore, preserving our environment needs to be of paramount importance. For this, it is imperative to detect pollution sources, understand contaminants and the effects these substances may exhibit. In this regard, the quality of air is of major significance, since earths’ organisms not only share but strongly depend on a common atmosphere. Air can be contaminated by pollutants that might either be man-made (anthropogenic) or of natural origin (non-anthropogenic), yet in both cases, air pollution might pose threats to the environment and the organisms living therein. Thus, reliable detection and quantification of pollutants are needed to avoid and counteract them. Further, the influence certain airborne substances might have on organisms, especially humans, needs to be understood for adequate risk assessment and treatment of health effects. In this review, we want to give an overview of the most important anthropogenic and non-anthropogenic pollutants and how these can be detected by using latest developments in the field of microelectromechanical systems (MEMS). Further, we want to discuss recent advances in modelling lung function on organ-on-a-chip (OoC) systems and their use in the assessment of health risks caused by airborne pollutants.
The air we breathe roughly contains 78% nitrogen (N2), 21% oxygen (O2), 0.04% carbon dioxide and various gases in trace amounts such as argon, neon or helium. In addition to water vapour, air also contains aerosols of solid fine particles or liquid droplets. There are countless examples of the change in air quality due to non-anthropogenic sources such as forest fires, volcanic activity, dust- and sandstorms, methane release from permafrost or radon gas that accumulates in confined areas. With the discovery of fire usage, humans started to burn wood to cook, keep themselves warm, keep away predators and develop technologies. Later, wood was replaced with fossil fuels such as coal and oil.
Burning these in industry or vehicles results in the emission of a number of different pollutants including sulfur, sulfur oxide, carbon monoxide and carbon dioxide but also many other toxic gases and small particles. The mentioned pollutants only represent a small choice of possible contaminants and sources. Yet, it is important to keep in mind that air quality can change in response to both, non-anthropogenic and anthropogenic contaminants. However, the amount and composition of airborne pollutants has changed due to anthropogenic activities, and might pose a challenge to the environment and thus human health.
The assessment of air quality helps to determine pollution sources and monitor contaminant levels. Recent advances in fabrication technologies gave rise to MEMS that are devices with dimensions in the micro- and nanometer scale. These devices are usually produced by the same technologies employed for the fabrication of microchips. Thus, fabrication of MEMS is fast, precise, relatively simple and cheap.
In principle, gas and particle sensors for air quality assessment based on MEMS technology are thought to be superior to other sensor types regarding selectivity, decreased power dissipation, lower operating temperature, and quicker response. These characteristics allow for the manufacture of small, cheap and mobile air monitoring devices enabling on-site real-time detection of pollutants. Further MEMS technology is robust enough to be employed in extraterrestrial space, allowing for a cost-effective pollution monitoring on a global scale by the employment of microsatellite grids.
The same advances in fabrication techniques that are used for the manufacture of MEMS can be utilized to create OoC systems. Here, cells are added into the system to recreate organ function or aspects thereof. The integration of sensors and the ability to observe the OoC with a microscope facilitate the real-time observation of biological processes in unprecedented detail. The lung is a rather sophisticated organ, with different compartments forming the conductive airways, while the alveoli facilitating the actual gas exchange. The distinct compartments of the lung have different structural and cellular characteristics.
Hence, most published OoC do not try to recapitulate the complexity of the whole lung, but rather the alveoli as the interface between air and bloodstream. Most notably, the stretching of the alveolar membrane through breathing motions can be simulated in latest lung-on-a-chip (LoC) developments. These lung OoCs were used to investigate asthma, chronic obstructive pulmonary disease, pulmonary edema or cystic fibrosis, thereby showing their potential for the recreation of physiologic lung function. The effect of exposure to airborne pollutants on lung cells was also investigated by using a device that was engineered to mimic respiration characteristics of a human smoking a cigarette. The cigarette smoke generated by the device was then transferred to the LoC to successfully recreate smoking-associated disease patterns. These advances show that LoC technology is in principle suitable for the risk assessment of airborne pollutants.
It is obvious that air pollution will remain an important issue in the future. MEMS technology will help to cost-effectively detect pollution sources with the possibility of collecting data on a global scale and the ability to reach secluded areas through satellite technology and hand-held devices, respectively. LoC technology will enable better estimates on the toxicity of pollutants, instead of solely relying on animal testing and conventional cell culture experiments.
Review article: Air Quality Effects on Human Health and Approaches for Its Assessment through Microfluidic Chips, Genes (Basel). 2017