# Air Quality Effects on Human Health and Approaches for Its Assessment through Microfluidic Chips
## 1 Introduction  
The planet Earth is the only known biosphere for human beings; it is, therefore, of paramount importance for human health to protect and preserve this environment. For this, it is imperative to detect pollution sources, understand contaminants and the effects these substances may have on our body after both short and long-term exposure (or in other words, to understand acute and chronic toxicity). There are various particles, chemicals, and gases that influence the quality of the air we breathe. Incomplete combustion of coal, diesel, or other fossil fuels, can lead to the formation of cancerogenic substances. While providing numerous useful and versatile compounds in high amounts, such as plastics or novel chemicals, the corresponding industry often introduces these materials without prior testing for the long-term effects on the environment and human health. One notorious example for the unintentional introduction of hazardous particles by the industry is the fabricationand further incorporation of asbestos into building materials [1]. Similar scenarios occurred with many chemicals that are classified as volatile organic compounds (VOC). More recently, the advent of nanomaterials helped to create many novel materials and to improve the properties of already existing ones. These materials include, but are not restricted to, carbon-derivates, organic and inorganic nanoparticles, 2D-materials such as graphene, and others. Here, not only the chemical composition, but also the size and shape of the nanomaterial, all have an impact on human health. These novel materials, before being released for public use, and thus, into the environment, need to be tested for their toxicity. This includes time and cost-effective physiochemical analysis of different pollutants, and subsequent investigations on possible health risks. In this context, recent advances in fabrication techniques might contribute, as they allow for the miniaturization of sensors and detectors, and the design of organ-like in vitro systems. The advance of microelectromechanical systems (MEMS), a basis for the fabrication of microchips, was initiated by the need to concentrate many functional structures within a small area [2]. The micrometer- and even nanometer-sized structures are made by different techniques, such as photolithography, injection molding, embossing, etching techniques, or 3D printing [3]. For the fabrication of MEMS, different materials such as silicon, metals, ceramics, or polymers can be used. The applications for the MEMS devices range from ultrasound transducers [4; 5] and Lab-on-Chip (LoC) [6] to rather sophisticated Organ-on-Chip systems (OoC) [7]. Usually, the chip itself represents the core of a given system, while the accompanying peripheral instrumentation setup can be much larger. For example, in air quality sensors, samples of air are first taken and then delivered to the chip component of the setup. The particle size and chemistry is then determined on the chip, providing an immediate readout. Similarly, cells ranging in numbers between one and a few hundred to thousands can be deposited and examined using MEMS technology. Here, the chip is part of a setup that usually involves a microscope, fluid control, and sensors, with their corresponding electronics, needed to deliver the readout of interest. For most portable devices, the chips are optimized towards an increase in energy efficiency. There are countless possibilities to utilize LoC and OoC systems. The established technologies already allow for fabrication and subsequent application, and are robust enough to be used in extraterrestrial space, for example in satellites [8]. For the use in space, MEMS allow minimizing various components down to a size range of a few micro- or nanometer-microsensors, actuators, accelerometers, heat controllers, microfluidic thrusters, microwave devices, satellite communication, and others.