**2. Sources of indoor air pollution**

Sources of indoor air pollution can be broadly classified into two as:


Anthropogenic sources of indoor air pollution are sources that originate from human activities for different purposes.

#### **2.1 Environmental Tobacco Smoke (ETS)**

It is also known as secondhand smoke exhaled by smokers. ETS contains a mixture of more than 7357 compounds, of which at least 40 of them are carcinogenic [9]. The only source of ETS is the combustion of tobacco products at home, vehicles, and office [10]. The combustion of tobacco products emits chemicals in the form of particulate matter (PM2.5 and PM10) and vapor form.

#### *2.1.1 Combustion of products*

Human beings throughout the world utilize biomass fuels to various degrees with the greatest proportion belonging to developing and economically disadvantageous nations. Solid fuels such as firewood, animal dung, and charcoal are the primary source of energy that accounts for 90% of consumption in sub-Saharan Africa for the purpose of cooking, heating, and lightening [11]. Other sources like gas water heater, gas cloth dryer, kerosene/gas space heaters are combustible products.

The combustion of those products can release gaseous pollutants most importantly carbon oxides (CO, CO2), nitrogen dioxide, and sulfur dioxide.

Carbon monoxide is asphyxiant gas that has a greater affinity to hemoglobin (oxygen transporter for tissue) than oxygen does [2]. This affinity results in the formation of carboxyhemoglobin and disrupts oxygen transport.

#### *2.1.2 Volatile organic compounds (VOCs)*

VOCs are compounds that emit gas at room temperature from certain solids or liquids [12]. Nowadays, pesticides are largely used in agriculture and food processing; emit small particles that can easily pollute indoor air quality. Moreover, air conditioners, hair spray building materials constitute a greater proportion of VOCs. Stationary

materials such as copy machines and printers, adhesives, and permanent markers can also contribute to VOCs.

Natural sources of indoor air pollution occur without human intervention.

#### *2.1.3 Biological contaminants*

Animal dander, molds, and dust mites are the common sources of biological contamination that give rise to particulate concentration and resultant indoor air pollution [4].

There are conditions that encourage the growth of those biological contaminants and their presence in the air:


#### *2.1.4 Heavy metals (airborne lead and mercury vapor)*

Although nowadays items become lead-free, older housing and furniture are coated with lead. Additionally, art and craft materials, automobile radiators, and solders are the main sources of lead that lead to pollutant concentration.

Mercury vapor: nowadays new paints have emerged as a concern to contain a high level of mercury in water-based paints in the form of phenyl mercuric acetate (PMA). The sprinkling of mercury by some ethnic or religious groups for ritual activities. Moreover, it is used for herbal medicine and botanic shops [12].

#### *2.1.5 Asbestosis and radon*

Both are known carcinogenic agents that arise from structural fireproofing and acoustic installation in floor and ceiling tiles. With the advancement of age, asbestoscontaining materials become damaged and disintegrate which gives rise to microscopic fiber into the air which increases the concentration of indoor air pollutants.

Radon: naturally occurring colorless, odorless, and tasteless radioactive gas that arises from the decay of radium or uranium. Earth-derived building materials and underground water especially, well water from private supplies are the major sources of radon pollution in households [8].

#### **2.2 Measurements of indoor air pollutants**

#### *2.2.1 Particulate matter*

Particulate matter (PM) is a term used to describe extremely small particles and liquid droplets in the atmosphere suspended in a gaseous medium. It is a complex mixture of particles including dust, soot, smoke, and liquid droplets which are generally small enough to be inhaled [13].

Based on aerodynamic diameters, particulate matter can be broadly classified into two


The concentration of particulate matter confined in the indoor environment can be expressed in two ways as:


Different instruments can measure the concentrations of particulate matter and provide either an average concentration for the sampling period or real-time monitoring concentrations [3].

Four major techniques can be employed to measure the concentration of PM.

#### *2.2.2 Gravimetric principle (filter-based samplers)*

The basic principle is collecting target particles on a filter. Air is allowed to be drawn with a certain flow rate with the help of a pump. Filters are going to be measured before (unloaded) and after (loaded) sampling keeping temperature and relative humidity to the standard condition. Simply, the difference between the two measurements gives us the mass of captured particulate matter. The mass concentration is then determined by dividing the collected mass by the known volume of air drawn through the filter by the pump. Accurate time-average mass concentration could be obtained for the period of sampling.

Impaction or centrifugal force is crucial for removing particles larger than the target aerodynamic diameter from the air stream.

#### *2.2.3 Microbalance principle (tapered element oscillating microbalance or TEOM)*

The principle behind this method is just putting a filter on the hollow element which is oscillating with its pattern when air passes through the filter. This oscillating frequency varies over time and the change of frequency is the measure of PM concentration build-up. It produces continuous data.

#### *2.2.4 Beta-ray attenuation principle*

After collecting the target particles on the filter, the mass of PM is determined by directing beta-rays from radioactive sources through the filter and the particles on it. The measure of PM mass is determined by the beam crossing the filter.

#### *2.2.5 Light-scattering principle (photometry)*

Usually, portable devices suck sample air through a chamber. Inside the chamber, sample air is provided with a narrow beam of artificial light. A photodetector measures
