Indoor Air Pollution

Abstract

Indoor air pollution becomes a public health hazard across the world. It originates from different sources such as the use of unclean fuel in developing countries for cooking, heating, and lighting purposes. Their use results in incomplete combustion. Carbon monoxide and other toxic gases are the primary result of incomplete combustion and can cause respiratory tract problems. Children and women who spent a large portion of their time indoors are the most vulnerable subpopulation.

1. Introduction

Indoor air pollution, also known as household air pollution is a state of increased concentration of air pollutants within and around a building or structure at a degree high enough to harm human beings and the ecosystem at large.

Air pollutants can arise from indoor activities like cooking, heating, and lighting using unclear fuels such as firewood, charcoal, animal dung, and kerosene [1]. The incomplete combustion of those fuels results in the production of carbon monoxide, nitrogen oxides, and sulfur oxides [2]. Other indoor activities like the application of insecticides, pesticides, air conditioners, and tobacco smoke can also contribute a large share of indoor particulate matter concentration [3]. Particulate matter is the complex mixture of dust, soot, smoke, and liquid droplet that suspends in the air in a small amount that can be inhalable.

Exposure to indoor air pollutants makes susceptible individuals breathe in those pollutants and activate as much as 15 sensory nerve receptors in the respiratory tract system that leads to reflex mechanism [4]. Pollutants that have a diameter of less than 5 micro-meter are candidates to reach alveoli by passing against physiological and mechanical barriers of the respiratory tract system [5]. After inhalation and availability in distal respiratory receptors, the pollutants cause acute respiratory infection most commonly pneumonia among young children and exacerbate asthma, chronic lung disease, and cancer among adults [6].

Although air pollution becomes a global concern that affects the population of the world in all spheres, indoor air pollution is a particular health risk for low-income countries [7]. Groups of sub-population such as infants, women, elderly, person with chronic disease, and urban residents who spent a large proportion of their time indoors, are at higher risk when compared to the general population [8].

Special locations including schools, homes, and workplaces (wood processing industries and cement factories) are sites with greater concerns. Ventilation and meteorological parameters such as temperature and humidity play a crucial role in the concentration of indoor air pollutants.

Indoor air pollution becomes a public health concern that results in a broad array of health problems ranging from reversible to irreversible damage to the body system. So, it is important to discover the source from which pollutants arise and develop skills on how to measure and monitor indoor pollutants to combat against its adverse health effect.

2. Sources of indoor air pollution

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

1. Anthropogenic (Man-made)

2. Natural

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 (PM_2.5 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:

• High relative humidity: that encourages dust mite population and hastens fungal growth on the damp surface.

• Inadequate exhaust for bathroom/kitchen generated moisture.

• Appliances such as humidifiers, dehumidifiers, and drip pans under cooling coins (refrigerator) support bacterial and fungal growth.

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, asbestos-containing 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

1. Coarse particulate matter (PM₁₀): particle matters having a diameter of greater than 2.5 micrometers and less than 10 micrometers.

2. Fine particulate matter (PM_2.5): particle matters of 2.5 micrometers and less.

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

• Particle number concentrations (count per cm³, L, m³)

• Particle mass concentrations (μg/m³ or mg/m³)

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 the amount of light scattering via sampled air-containing particles. These devices are factory calibrated, contain own pump and storage. It can provide continuous data.

Generally, particulate matter (PM) has to be measured at the different locations inside the living house such as the kitchen, where most of the emission comes from fuels, living room, and outdoors for comparison. Measurement of PM should be:

• Approximately 1 meter away from the edge of the combustion zone (from the stove approximates the edge of the active cooking area.

• At the height of the sitting position (1 meter) above the floor which is a breathing zone for sitting women while cooking and children.

• At least 1.5 meters horizontally away from windows and doors, where possible [14].

The outdoor particulate matter has to be measured at the sampling location of height about 2 meters from the ground level, a minimum of 2 meters away from obstructions like the wall of the house, 10 meters from trees, 5 meters from the chimney, and 5 meters from the edge of the nearest traffic lane.

WHO set a standard for air quality monitoring for different pollutants, among which PM is the most significant. According to the organization, the standard provides the maximum and interim target values. Interim targets are proposed as incremental steps in the reduction of air pollution and are intended for use in areas where pollution is high 1] (Table 1).

Pollutants	Measurement	Averaging time	Interim Target				AQG level
			1	2	3	4
PM2.5	μg/m^3	Annual	35	25	15	10	5
		24-hour	75	50	37.5	25	15
PM10	μg/m^3	Annual	70	50	30	20	15
		24-hour	150	100	75	50	45

Table 1.

WHO standard concentration of particulate matter at different stages of measurement.

2.3 Carbon monoxide (CO)

It is a colorless, odorless, and tasteless gas that is poorly soluble in water. Relatively high carbon monoxide levels have been measured inside homes with faulty or unvented combustion appliances, particularly if the appliances have been used in poorly ventilated rooms.

2.3.1 Analytical method

The reference method for the measurement of carbon monoxide concentration is based on the absorption of infrared radiation (IR) by the gas in a non-dispersive photometer [15]. CO absorbs IR radiation maximally at a wavelength of 4.7 micrometers (μm), which is in a spectral range where few other atmospheric species absorb to interfere with accurate quantification.

Carbon dioxide and water vapor can make major interference with the measurement of carbon monoxide. Removal of water vapor from the sample air is necessary to avoid positive interferences in the determination of CO concentration and is achieved by a permeation tube or drier that selectively removes water vapor from the sample gas without removing CO.

The concentration of CO can be determined using the Beer–Lambert law which relates the concentration of an absorbing species to the degree of light attenuation. The analytical method is suitable for stable installations at fixed-site monitoring stations.

Currently, potable carbon monoxide individual-level exposure measurements become familiar. Measurements of personal exposures are based on the electrochemical reactions between carbon monoxide and deionized water, which are detected by specially designed sensors.

The following guideline values (ppm values rounded) and periods of time-weighted average exposures have been determined in such a way that the COHb level of 2.5% is not exceeded, even when a normal subject engages in light or moderate exercise:

• 100 mg/m³ (90 ppm) for 15 minutes

• 60 mg/m³ (50 ppm) for 30 minutes

• 30 mg/m³ (25 ppm) for 1 hour

• 10 mg/m³ (10 ppm) for 8 hours

2.4 Estimation and measurement of VOC emissions

VOC can be measured at stationary level and individual concentration (personal level) [16].

2.4.1 Total VOC concentration measurement techniques

• Flame ionization detectors (FID)

• Catalytic oxidation and non-dispersive infrared absorption

• Photoionization detection (PID)

2.4.2 Individual VOC substance concentration measurement techniques

• Gas Chromatography (GC)

• Non-Dispersive Infrared Spectrometry (NDIR)

• Fourier Transform Infrared absorption (FTIR)

2.5 Health effects of indoor air pollutants

Indoor air pollutants that are emitted from different sources occupy the living and get entrance to our internal body systems usually through inhalation. Although the target organs may vary from one pollutant to another, the most common are the lungs, eyes, and nervous system. It is important to understand the kinetics and metabolism of pollutants for better knowledge of their effects on the human body system.

2.6 Health effect of carbon monoxide

Kinetics of carbon monoxide starts with inhalation of the pollutant and diffuses to the alveolar membrane of the lung. During a gas exchange that takes place in capillaries, the CO gas can be exchanged in the same way oxygen does. After its bioavailability in plasma, quickly binds with hemoglobin (Hb) at rate 200–300 times more than oxygen [15]. It can equivocally compete for hemoglobin binding sites with oxygen. Unlike oxygen, the bond is stronger and lasts longer. The formation of carboxyhemoglobin (COHb) makes the transport of oxygen difficult for the body system and results in a state of hypoxia. The primarily affected body organs with reduced oxygen delivery are the myocardium (muscle of the heart), brain, skeletal muscles that employ exercise, and developing fetus. At low levels, symptoms of CO exposure include fatigue, headaches, and dizziness, but in higher concentrations, it can lead to impaired vision, disturbed coordination, nausea, and eventually death.

2.7 Health effects of biological agents at particular level

It is important to broadly classify the health effects into three categories as follows:

1. Infection: direct invasion of tissues and organs by particulate-level biological agents like dust mites, molds, and animal dander causes pathogenic infections.

2. Hypersensitivity disease: inhalation and absorption of certain pollutants can evoke and activate the specific immune system.

3. Toxicosis: biologically produced toxins such as mycotoxins (fungal metabolites) can cause direct toxic effects.

2.8 Health effect of pesticides

Nowadays, pesticides become input for effective agricultural practice and are widely used across the urban and rural communities. Inhalation, ingestion, and skin absorption are the common exposure routes of particles in pesticides. Pesticide exposure is associated with adverse health risks, including short-term skin and eye irritation, dizziness, headaches, and nausea; and long-term chronic impacts, such as cancer, asthma, and diabetes.

A case–control study has shown the association of breast cancer with exposure to certain pesticides in agricultural sectors [1].

2.9 Health effect of heavy metals

Heavy metals like mercury vapor, lead, chromium, and cadmium residue are known for their bioaccumulation and biomagnification properties. Bioaccumulation is the kinetic property of the chemicals in which metabolism and excretion of certain elements are difficult and result in accumulation in specific tissues and organs of the human body system. An increase in the concentration of those heavy metals when they came across the complex food chain is termed as biomagnification.

Studies have proved that Mutagenic and carcinogenic effects are common health effects related to heavy metals [4]. Moreover, brain damage, respiratory illnesses, cardiovascular disease, and deaths can be attributed as a result of it.

2.10 Health effect of particulate matter (PM)

Exposure to particulate matter is considered as a public health hazard. It can primarily attack the lung followed by the heart and eye. It has acute and chronic effects.

Acute exposure to those particles can cause:

• Persistent cough

• Shortness of breath

• Chest tightness

• Eye irritation

• Irregular heartbeat

• Nonfatal heart attacks

Chronic exposure to the particles can cause:

• Decreased lung function

• Recurrent respiratory infections among children

• Premature death in people with heart or lung disease

• Aggravated asthma

A cross-sectional study has shown the association of acute respiratory infections with particulate matter concentration (PM_2.5 and PM₁₀) from biomass fuel exposure in an indoor environment [14].

Strategic approaches to reduce indoor air pollution.

Indoor air pollution become a public health issues that need urgent interventions to save the life of millions of people and keep the environment from damage. Different approaches have been presented at international, regional, and national levels. Implementing those approaches is vital to coping the existing situation.

2.11 Energy sources development

It is impossible to achieve indoor air quality without the provision of clean energy. A large proportion of the world relies on unclean fuel that results in incomplete combustion. Energy preference largely depends on the economic power of the nation. Policies and strategies that enable a nation to use natural resources like water for hydroelectric power, which is environmentally friendly and helps transform energy to clean have to be implemented in developing countries.

2.12 Pollution prevention principles

As a human being we can prevent the occurrence of pollution before it happens and reduce consecutively its impact if it occurs. Pollution can be prevented at the source, path, and receiver (usually individuals). Prevention at source means avoiding pollution occurrence in the first place by either eliminating, banning, or restricting the usage of hazardous chemicals at commercial levels. Pollution prevention at the path includes proper ventilation of living rooms which is expressed in terms of window floor ratio. Finally, prevention at the receiver can be using personal protective equipment like wearing masks where the concentration of pollutants is higher.

2.13 Taxation

Selective taxations are crucial for tobacco products in order to minimize environmental tobacco smoke at household level. Additionally, environmental taxation on leaded petrol has brought great influence and was successful [17].

2.14 Information dissemination, education and training

Adequate information has to be disseminated for policymakers to design appropriate strategies to address the issue. Moreover, designers, building, and construction professionals have to be informed about the raw materials they use as construction input.

3. Conclusion

As a result of human activity for the sake of earning a living and urbanization, humans exploit nature and use different raw materials that can end up in the generation of pollutants that affect the health of human beings. The effect is significant among occupants of the household that spend a large portion of time indoors usually younger children and women. Regular monitoring of pollutants using real-time monitors is crucial to understand the degree of risk and designing appropriate prevention strategies.