**4.1 Health impacts resulting from PM and its constituents**

Growing number of studies in the last few years provided evidence linking disease and adverse effects to extremely low levels of PM and its toxic components. Many components for instance organics such as PAHs, inorganics such as heavy metals are known carcinogens and are responsible for numerous adverse health effects in humans. Apart from this, exposure to PM has been linked to increased hospital admissions, carcinogenicity, developmental disorders, nervous system effects, respiratory symptoms, cardiovascular diseases, decreased lung function and premature mortality [35–40]. Recent finding showed the presence of nanoparticles on the foetal side of the placenta indicating that the placenta barrier can be easily penetrated by the PM resulting in exposure to foetus [41]. This indicates that the extent of toxicity of PM is not only limited to the adults but also to the foetus as well. Moreover, long term exposure to PM pollution renders a population more vulnerable to COVID-19, increased hospitalisations of patients with predisposed asthma or other respiratory ailments were also reported. Hence, the impact and toxicity of PM and its components depend on various factors

**Figure 3.** *PM and human health impacts.*


#### **Table 4.**

*PM component and its health effect.*

including their solubility in water, residence time, elemental composition, particle size and chemical reactivity; local environmental conditions- season, wind speed, topography etc.

At present, research studies based on population data do not provide enough evidence to identify differences in the effects of particulate matter with different chemical constituents [37]. However it should be noted that the evidence for hazardous nature of particulate matter generated through the combustion process, from mobile and stationary sources, are more consistent than that from other sources [38]. The black carbon fraction of PM2.5 that is resulted from incomplete combustion has great concern towards their contribution to detrimental effects on human health as well as on climate. Other constituents of particulate matter attached to black carbon are seen as responsible for various health effects, such as PAHs that are well known carcinogens and toxic to the cells, also metals and inorganic salts. It has to be noted that diesel engines exhaust, consisting mostly particulate matter, has been classified as carcinogen (Group 1) to humans by the International Agency for Research on Cancer [39]. This also includes some PAHs as well as some solid fuels used in household (**Figure 3** and **Table 4**) [40, 42].

#### **4.2 Respiratory effects**

Respiratory system is mainly affected by all types of air pollutants. High levels of sulphur dioxide, nitrogen oxides and ozone are linked to symptoms such as nose and throat irritation, cough, chest discomfort due to narrowing of airways, increased mucous production on the walls of upper airways following which inflammatory reactions may occur leading to asthma and more severe condition COPD [42, 43]. People with respiratory ailments are more sensitive to air pollution exposure.

The human health risks of PM are attributed to their deposition and transportation in the human body. The particles are deposited in the lungs by the process of impaction, interception, sedimentation and diffusion. In general, larger PM fractions (> PM2.5) are mostly deposited in the upper respiratory tract by means of impaction. Smaller size PM (< PM2.5) can deposit deeper into the lower airways and alveoli depending on the flow rates and diffusion, and may be transported to other tissues and organs via bloodstream. Several studies have also provided the evidence that exposure to smaller particles can result in serious health effects. Although, the impact is variable depending upon the composition (heavy metals, PAHs act as carcinogens), concentration and duration of exposure to the particles. Exposures to PM bound heavy metals (arsenic, nickel, lead etc.) are responsible for asthma, emphysema and even lung cancer [44].

Although in vitro studies on human health due to PM pollution are numerous, there are relatively few that focus on the epidemiological aspects. Studies are scarce regarding investigation focusing on the mechanism of PM toxicity at cellular and molecular levels (**Figure 4**).


**Figure 4.** *PM and associated respiratory effects.*
