**4. Animal models of PM studies**

Experimental studies to evaluate air pollution-induced oxidative stress in humans are challenging to perform for ethical and population heterogeneity reasons [29]. Thus, several experimental studies have shown the effects of exposure to air pollution [18, 20, 57–60], for which many models are used to assess the degree of exposure versus the effect produced. Also, the studies of exposure to pollutants in animal models, which mimic exposure to air pollution in the human population, enable the study of pathophysiological mechanisms by which the body develops numerous diseases related to air pollution.

In this way, many in vivo exposure studies models in animals have been published, with different exposure models to atmospheric pollution. Inhalation methods are closer to mimicking the reality of human exposure, such as inhalation of concentrated PM (CAP) or inhalation of environmental air particulates.

In the CAP method, the exposure system uses the principle of the condensational growth of the ambient particles followed by virtual impaction to concentrate the aerosol and allows the particles larger than 2.5 μm in aerodynamic diameter removed at the concentrator inlet, and the remaining aerosol to be concentrated by inertial separation techniques that dispose of most of the carrier air, which enables delivery of concentrated streams of real-world particles to human subjects or laboratory animals via whole-body exposure [60].

The environmental air particles propose, the exposure of the animals to PM in a chamber, where the air is propelled from the environment to the inner area of the chamber, is frequently used [61–63]. In this model, the experimental groups are divided into filtered air (with a series of filters to prevent pollution from entering) or unfiltered (exposed/polluted group). Alternatively, this model can be performed by uptake urban air using a concentrator of atmospheric pollution particles [64–66]. The last one was used early in toxicological studies, using an atmospheric pollution particle concentrator developed by Harvard University [66]. The advantage of the chamber protocols is to submit the organism to a "real word" air pollution context. On the other hand, since the urban pollutants may present a significant variability between the days of the exposure, at least in terms of concentration and composition, the animals are exposed to days of higher levels and days with low levels of PM in the chamber [61, 62]. As a result, the biochemical and molecular outcomes in this model may also present a significant variability and the interpretation of the results can be specific for urban areas.

Other studies used intratracheal and intranasal instillation methods [26, 67, 68]. Intratracheal instillation should generally be considered a method for a single exposure of the lungs to characterize potential toxicity [69]. The intratracheal instillation

#### *Environmental Particulate Air Pollution Exposure and the Oxidative Stress Responses: A Brief… DOI: http://dx.doi.org/10.5772/intechopen.101394*

method is often used to expose animals to both soluble and insoluble particles, and it is a relatively inexpensive method that allows instantaneous administration directly into the trachea of known concentration of the pollutant under test directly to the lung [70]. In the intranasal instillation model, the handler restrained the animal (suspended by the cervical region, held with the hand on the back). Then the suspension in liquid (with a known concentration of pollutant) is administered directly into the animal's nostril, that by apnea reflex, inhales the suspension [70, 71]. Thus, the advantage of the intratracheal or intranasal protocols is to submit the organism to a controlled pollution context in terms of dose and frequency of exposure, adequate for toxicological studies and mechanistic studies. Also, since the particles may present a significant variability in terms of the composition, dependent on the source of particles used in the study (e.g., metal composition variability), intratracheal and intranasal protocols allow that, even if the animals are exposed to the same dose, but with different source particles, the studies may reveal different responses. However, as limitations, these procedures may not represent a "real world condition".

When compared to inhalation procedures, instillation is also easier, less expensive, and incurs less health risks to the lab staff. Nonetheless, inhalation is a more realistic physiological approach since it better represents the natural route for PM exposure, and CAP inhalation remains the main one, as it is closer to simulating reality environmental conditions and physiological animal responses in laboratory PM administration [72].

A study by Curbani et al. [72] highlights that studies with intratracheal or intranasal instillation models PM concentrations were three orders of magnitude higher than the environmental ones found in megacities through acute and sudden exposures. And inhalation exposure protocols were closer to ambient PM concentrations, being one or two orders of magnitude higher than the PM concentrations found in megacities. Researchers must be aware, since, the discrepancy is a result of the experimental conditions, where most protocols are planned to reduce the exposure time and increase the PM concentration, thereby attempting to achieve the same result as long time of exposure and low PM concentrations [72].

The models described above are complementary in the information about environmental pollution's impact on health. The development of models to assess exposure to air pollution within cities, to attribute the health risks produced is a priority for future research. The mechanisms of harmful health effects related to exposure to pollution are not fully elucidated in clinical and epidemiological studies, raising questions about how pathologies develop and why exposed individuals become more susceptible to certain conditions. The integrative view of epidemiological, clinal, and animal studies is required to elucidate the fundamental problem of air pollution. To reach this goal, animal models that mimic exposure to air pollution effects are appropriate to elucidate the pathophysiological mechanisms by which the body develops numerous diseases. These studies are complemented by in vitro, in silico models to explain toxicological aspects. Studies using these exposure models have effectively related the damage caused by air pollution to the development of chronic diseases, whether respiratory, cardiometabolic, or neurodegenerative, highlighting mainly the physiological effects produced for the aggravation of these conditions [73].

### **5. Conclusion**

Experimental and epidemiological studies have shown evidence of the harmful effects of exposure to air pollution, especially by PM. The effects caused by such

particles depend on their composition, the exposure time, and the particle size, with emphasis on fine and ultrafine particles due to their multisystemic action. Animal models of PM exposure revealed clearly the pathophysiological mechanisms at the systemic and cellular levels, and the choice of the experimental protocol needs to be clear in terms of the source of the pollutant, dose, and representability of toxicological or real word aspects. Complementarily, animal studies highlighted that the increase of air pollution levels induces responses by oxidative stress, which is an initial mediator of inflammatory processes and helps to elucidate such mechanisms and contribute to the establishment of stricter policies to control emissions and air quality.
