Most developed countries have houses built on a concrete base with a wooden framework and asbestos sheets. Building material is further packed with fire-retardant material to reduce the fire hazards in such houses. Flooring usually is made of polished and varnished wood. Many houses also feature vinyl flooring or carpeted flooring. Clean liquefied petroleum gas (LPG), natural gas, or electricity is used for cooking. Many of these houses are fitted with air conditioners to maintain a comfortable room temperature. Many houses feature an electric furnace or a wood fireplace for heating, more so in areas prone to snowfall. To ensure effective thermal conditioning of the house, insulation material is used extensively. Upholstery such as heavy drapes, thick sofa covers, decorative throws, and pillows are a regular feature in these houses. Varnished and polished wood is used for furnishings. Very often, particle board material is used for furniture for its ease and economy of use. Bedding material used in these households includes down feathers, coir, and foam. The use of scented candles, room fresheners, potpourri, frankincense, and so on is a usual occurrence. In this era of advanced technology, most of these houses are equipped with modern gadgets such as printers, copiers, and fax machines. Improved personal hygiene also involves the extensive use of cleaning agents, perfumes, and deodorants, regular painting and varnishing, floor and furniture polishing, and so on. Insect repellents are used in many of these households in order to keep the home environment clean.

Most developing countries, on the other hand, use stones, bricks, concrete, and cement to build their houses. Although the use of air conditioning is on the rise in warmer developing countries, most houses rely on natural ventilation through open windows and shutters. Overcrowding and increased industrialization have led to housing within close vicinity of industries and heavy traffic-dense roads. The poor socioeconomic strata of society in these countries continue to live in ill-ventilated and ill-lit houses made of bamboo, wood, crop residues, tin sheets, and sometimes cloth. Biomass for cooking, heating, and lighting is used widely in these parts of the world. Paints and whitewash are commonly used in these houses. Insect infestations remain a major problem in developing and rural houses, and repellents are widely used. The use of fragrances through air fresheners, frankincense, scented candles, and potpourri is a rising trend. Increasing demands for improved housing have led to a growth spurt of housing complexes with a faster turnover time period. This in turn has affected the quality of the finished housing, leading to poor quality of the construction material, faulty electrical and plumbing work, and an overall poor quality of housing ( Figure 2).

One of the major sources of household air pollution, especially in developing countries, is fuel used for cooking as well as heating practices. Homes from developed countries and many houses in the developing world use electricity, natural gas, or clean LPG for cooking, whereas houses in rural communities and some houses of the developing world use biomass fuel for cooking.

Natural gas is primarily methane, whereas LPG is primarily propane or a mixture of propane and butane. Natural gas requires less air for combustion (an air-to-gas ratio of 10:1). LPG, on the other hand, requires more air for combustion (an air-to-gas ratio of 25:1), releasing almost three times the energy released by burning natural gas (93.2 MJ/m ³ through LPG versus 38.7 MJ/m ³ through natural gas). LPG is relatively denser than air, which is further denser than natural gas (1.52:1:0.55). Gas leakage when using LPG therefore tends to settle in the household air at human levels, whereas leakage of natural gas ascends toward the ceiling, reducing health effects. Burning of natural gas not only produces a variety of gases such as sulfur oxides, mercury compounds, and particulate matter but also leads to the production of nitrogen oxides, primarily nitrogen dioxide.

Biomass fuel includes wood, crop residue, animal dung cakes, and wood charcoal. Approximately, 3 billion people, or half the world’s population, use biomass for cooking or heating across the globe and burn about 2 million kilograms of biomass every day. China alone is responsible for 420,000 annual deaths due to indoor air pollution caused by the use of solid fuels ⁴ . These homes have very high levels of particulate matter and gaseous air pollutants such as carbon particles, iron, lead, cadmium, silica, phenols and free radicals, carbon monoxide (CO), nitrogen dioxide, sulfur dioxide, formaldehyde, hydrocarbon complexes, and other inorganic and organic substances which include polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds, and chlorinated dioxins. Various studies ⁵ have shown that, on average, the levels of particulate matter of less than 2.5 microns in mean aerodynamic diameter (PM _2.5) in such houses range from 500 to 1,500 µg/m3, which is very high, as the permissible indoor level of PM _2.5 according to the WHO ranges between 10 and 50 µg/m ³ in a 24-hour mean value ⁶ . High levels of CO, especially during the burning of charcoal, are also produced. However, burning of wood produces the least amounts of PM _2.5 and CO among the biomass fuels ⁷ .

The method of cooking also has an impact on the levels of particulate matter released in the air. Stir frying, deep or shallow frying, charbroiling, roasting, and grilling have different emission levels of particulate matter. The type of meat, the amount of fat in the meat, and the type of oil used for cooking also determine the emission levels. Meat charbroiling, for example, emits large quantities of particulate matter of 0.1 to 0.2 µm in mean aerodynamic diameter ⁸ . Meat frying and charbroiling contribute to about 21% of the particulate matter emitted. Regular meat emits about 40 g of particulate matter per kilogram of meat when charbroiled, whereas lean meat emits about 7 g of particulate matter per kilogram of meat when charbroiled ⁸ . When subjected to frying, the same meat emits about 1 g of particulate matter per kilogram of meat. Oil used in cooking emits a significant amount of PAHs, which further add to the household air pollutants. Stir frying, a popular cooking style in Chinese cuisine, has been shown to emit particulate matter ⁹ ranging from 300 to 1,700 μg/m ³.

Smoking tobacco in any form within the confines of a house is a major source of household air pollution. Globally, there are approximately 1.1 billion smokers, a number which is steadily on the rise. Cigarette smoke contains 7,357 different chemical compounds such as benzene, CO, PAHs, heterocyclic amines, cyanide, formaldehyde, terpenoids, phenols, nicotine, and heavy metals. Burning of tobacco also emits considerable amounts of PM _2.5 (burning one cigarette emits 7 to 23 mg of PM _2.5) ¹⁰ .

Tobacco smoke could be first-hand, second-hand, or third-hand smoke. A person who smokes within the confines of his house is exposed to the smoke himself (first-hand smoke). Other occupants of the house inhaling these fumes but not actually smoking are exposed to second-hand smoke. The particles emitted during smoking settle on the furnishings, hair, clothes, and the floor. These particles remain suspended in the household air for quite a while even after the primary smoker has exited the premises. This constitutes third-hand smoke.

Temperature control may involve both heating and cooling of the household environment. Houses in developed countries and some houses in developing countries use air conditioning for temperature and humidity control. To ensure effective conditioning of the house, insulation material and drafting control are used extensively. Preventing the temperature-controlled air from escaping the closed environment leads to poor ventilation in these houses and further causes accumulation of the particulate matter pollutants inside the house ¹¹ . Furthermore, inadequately cleaned air conditioning units are breeding grounds for various fungi and bacteria.

Opening windows is a form of natural ventilation. Ambient air pollutants containing copper, iron, potassium, nickel, silicon, vanadium, and zinc, with a particle size of between 2.5 and 10 microns, and ozone gas easily find their way into these houses through the natural ventilation, in turn contributing to household air pollution ¹² . In particular, houses that are close to traffic-heavy roads, which are populated with vehicles using diesel as fuel, tend to have a higher load of household air pollutants. Diesel exhaust particles can settle on the pollen of roadside trees and consequently find their way into an open household ¹³ . Carbon particles, soot, diesel exhaust-laden pollen, and organic particulate matter are therefore found to be in very high concentrations in these houses which use natural ventilation.

Biomass has been used for heating in several parts of the world. Burning of biomass emits not only PM _2.5 but also high levels of PM ₁₀ (particulate matter of less than 10 µm in mean aerodynamic diameter) up to 5,000 µg/m ³ (limits permissible by the WHO are 100 µg/m ³ as a daily average). Novakov and Corrigan showed through their study on thermal characterization of biomass smoke particles that black carbon is in fact a biomarker of biomass fuel combustion ¹⁴ . Burning of charcoal also emits a substantial amount of polyaromatic hydrocarbons and particulate matters yet considerably lower than burning wood ¹⁵ .

The developing and rural world is still grappling with communicable and infectious diseases, usually borne by vectors such as mosquitoes, insects, and ticks. One of the biggest menaces is through mosquito-borne diseases such as malaria and dengue. In an effort to curb the growing mortality and morbidity rates of these mosquito-borne illnesses, mosquito control is a burning need. This is done by the use of chemical repellents. The most widely and commonly used repellent is the mosquito coil. Approximately 2 billion people across the globe use mosquito coils to ward off the dangers associated with mosquito-borne illnesses. A standard mosquito coil is made up of 0.1% of the active repellent pyrethroids, whereas the remaining 99.9% contains binders, resins, and flammable material such as coal dust and coconut husks. Twelve million such coils are sold every year in the world. The coil is ignited and allowed to smoulder for 6 to 7 hours. Ideally, the coil is meant to be lit and smouldered in a room with windows and doors closed to get the most desired effect. Previous studies have shown that burning of one such coil emits particulate matter equivalent to burning 100 cigarettes and PAH equivalent to burning 50 cigarettes. We have recently shown that levels of PM _2.5 and CO are 2,200 times and 10 times the limits permissible by the WHO, respectively, when the mosquito coil is burnt with doors and windows closed. Improving ventilation by opening the windows but retaining closed doors reduced the pollutant levels markedly but remained at 500 times and twice the limits permissible by the WHO ¹⁶ . Only when both the doors and the windows are kept open do the pollutant levels drop down to the safety limits. However, this defeats the use of the mosquito coil to control the mosquito menace. Other forms of mosquito repellents such as vaporizers, sprays, ointments, and medicated papers do not produce as much particulate matter but produce gaseous air pollutants that are irritants to the airway mucosa ( Figure 3).

In regions where malaria or mosquito-borne diseases are prevalent, household spraying with dichlorodiphenyltrichloroethane (DDT) or pyrethroids is very common. Repeated spraying of these chemical repellents leads to accumulation of these toxicants in the house. Interestingly, house dust concentrations serve as an excellent metric for estimating concentrations of insecticide exposure within the house. South African researchers showed a markedly high concentration of residual DDT in homes 2 months after spraying the insecticide (910 versus 1.3 μg/m ² in homes not sprayed) ¹⁷ . Similarly, a study from Mexico showed higher levels of residual insecticides as late as 3 years after spraying houses with DDT (30.8 μg/g of house dust versus 0.7 μg/g of house dust in homes not sprayed with the insecticide) ¹⁸ .

Poorly ventilated homes tend to accumulate kitchen odours as well as concentrate the household air pollutants. This warrants the use of perfumes and scents to improve the level of hygiene in the house. Improved hygiene involves the use of cleaning agents, perfumes and deodorants, scented candles, and so on to make the house more cosy and comfortable. Steinemann ¹⁹ studied 37 commonly used consumer products, which include air fresheners, laundry products, personal care products, and cleaning agents, to identify the volatile organic compounds released by them. Interestingly, this study identified 156 different volatile organic compounds, of which the US Food and Drug Administration classified at least 42 as toxic or hazardous. Similar observations have been made by other researchers as well ²⁰ .

Almost all religions resort to using some form of fragrances in their everyday religious activities. Hindus and Buddhists burn incense sticks and fragrances, offer oil-lit lamps, and burn smoke sticks called dhoop during their daily prayer service. Hindu marriages often involve the burning of a sacred fire with wood and animal dung cakes for at least 2 to 3 hours per ceremony. There are 3 million places of religious worship in India alone, and 10 million marriages are performed there each year. Similarly, Christians burn candles during prayers and more so during Easter and Christmas. Islamic homes tend to use fragrances in the form of Bakhoor and Oudh, which emit fragrances when placed over hot charcoal. Various studies have reported that toxic levels of air pollutants are emitted when these fragrances are burnt. The incense sticks called agarbattis, made of finely ground fragrant material, are bound with binders, usually makko, around a supporting bamboo stick. India alone sells incense sticks worth 225 million USD per year and this has been growing annually at 10%. Among the Chinese, 76.9% currently burn incense at home every day and over 90% of the population has been using these for over 20 years. Burning of these fragrances emits high levels of PAHs, benzene, nitrous oxide, and CO ²¹ ( Figure 4). Similarly, burning of candles produces PM _2.5 and PM ₁₀ to the tune of 1,200 and 200 µg/m ³, respectively ²² .

Paints and varnishes used in houses emit significant amounts of volatile organic compounds, increasing the burden of household air pollutants. Particulate board furniture is also responsible for emission similar to that of volatile organic compounds largely because they are held together by adhesives that emit volatile organic compounds. Insulation material used in buildings has also been implicated in the emission of volatile organic compounds ²³ . Asbestos used in sheets in the construction of houses leaves the finished houses with fine asbestos dust particles. Similarly, silicon particles are found in abundance in houses made of bricks and cement. Improvements in technology have led to digitalization in houses. Every household today has hi-tech electronic gadgets such as computers, tablets, iPads, printers, and mobile phones. These have been implicated in increased levels of ozone in the houses. Polybrominated diphenyl ethers (PDBEs) are used as fire retardants in foam-containing furniture and electronics. These emit PentaPDBEs and DecaPDBEs in minute quantities which contribute to household air pollutants ²⁴ .

Faulty plumbing, alone or coupled with weather conditions, has led to an increasing incidence of indoor wall dampness ²⁵ . These walls form an ideal environment for the growth of fungi such as Alternaria, Aspergillus, Cladosporium, and Penicillium ²⁶ . Along with fungi, damp mouldy walls are also a breeding ground for several species of Gram-positive and Gram-negative bacteria which include Streptococcus, Micrococcus, Staphylococcus, Mycobacterium, Nocardia, and Streptomyces ²⁷ . Budding spores of these fungi, microbial particulates, volatile organic compounds, mycotoxins, and endotoxins of the bacteria contribute a great deal to household air pollution ²⁸ .

Many household air pollutants, such as PM _2.5, benzopyrene, lead, ozone, nitric oxide, sulfur dioxide, organophosphates in household pesticides, and tobacco smoke, have the potential to disrupt the endocrine system of humans as well as animals. These compounds have been termed “obesogens”, as they lead to metabolic syndrome and obesity ²⁹ .

Many pathogens such as bacterial droplets, viral droplets, and fungal spores remain suspended in the household air after an infection in the family. These droplets settle on household surfaces, acting as fomites for the spread of infection.

Table 1 illustrates the various sources of household air pollution.

Major interventional strategies need to be implemented at various levels to help curb this growing menace of household air pollution and its detrimental effects on health. The medical profession is attempting to improve the situation through increased research on the effects of household air pollution on health. Spreading this research through continued medical education will help disseminate this new knowledge among practitioners, increasing awareness about household air pollution and its health effects. Consolidating nationwide data by all countries will help generate regional data regarding household air pollution as different regions have varied causes and subsequently diverse strategies to implement changes to reduce this growing menace. Governments must formulate policies to bring about positive changes through behavioral interventions and cost-effective methods to improve fuels used for cooking and heating. For example, developed countries have now placed a ban on PentaPBDEs used in fire-retardant foam-containing furniture and electronics. This has helped reduce the levels of PentaPBDEs in their houses. The media can create tremendous public impact and aid the government and the medical profession in creating awareness about household air pollution among the general public.

The use of adulterated fuels and biomass for cooking and heating is one of the most important contributors to household air pollution. Advocacy to use cleaner fuels will assist in curbing this menace to a limited extent ⁶² . One of the major deterring factors towards using cleaner cooking fuels is the economic burden of cleaner fuels. Improved cook stoves have been widely advocated to help reduce the emission from poor-quality cooking fuels ⁶³ . Locally fabricated top-lit updraft stoves used in Uganda ⁶⁴ have significantly increased the efficiency of wood used, producing less smoke and more efficient and faster cooking. They also end up producing charcoal. However, cost remains an issue. Other forms of improved cook stoves have also been implemented. Although the use of these improved cook stoves helps reduce the particulate matter produced, this load remains well above the WHO safety limits ⁶⁵ . An 85% reduced exposure to particulate matter is required to achieve a desired clinical effect. The use of cleaner fuels such as LPG would reduce the load of household air pollutants to a great extent. A study conducted in Sudan reported a significant reduction in the levels of particulate matter (51–80%) and CO (74–80%) in homes that used LPG versus those that used biomass fuel for cooking ⁶⁶ . Widespread adoption of natural gas as a source of fuel helped reduce infant mortality by 4% in Turkey ⁶⁷ . In fact, certain governments have requested that their affordable populations forego their subsidy on LPG in order to provide for the disadvantaged sections of their societies. However, implementation of widespread use of LPG requires a colossal effort from healthcare providers, healthcare policy makers, local governments, industry, and, most importantly, the general public. Economic barriers for the use of LPG need to be sorted out through government policies that are more liberal than the existing ones ⁶⁸ .

In cases where improved cooking fuels are not feasible, every attempt must be made to improve the ventilation in these houses. Poor ventilation has been associated with a 49% increase in the risk of lung cancers ⁶⁰ . Implementing building guidelines and enforcing them may provide a solution to ensuring better ventilation in homes. Improving ventilation and the use of cleaner fuels help in reducing the rate of lung function decline as well. Improving ventilation may need additional behavioral interventions. People need to be educated about the efficiency of cleaner fuels, the use of traditional cooking fuels outside the home, the use of elevated kitchen platforms to facilitate quicker exit of biomass fuel smoke, the use of a long-stemmed chimney, or the addition of windows or doors to the home. A Sri Lankan study ⁶⁹ reported that households using a chimney along with traditional cook stoves had a PM _2.5 level of about 70 μg/m ³ but that households using traditional cook stoves without a chimney had PM _2.5 levels of about 310 μg/m ³. However, local traditional cultures pose a challenge to behavioral interventions. Traditional methods of cooking, hampered use of cooking processes with improved cook stoves, non-conformity of improved cook stoves to traditional cookware, changes in flavor of food cooked on improved cook stoves, and lack of familiarity with the type of fuel used in improved cook stoves are the most important reasons for failure to implement improved cook stoves. A noteworthy case of behavioral intervention was reported by the Timor community of Indonesia ⁷⁰ . This society had a custom of enclosing a newborn baby and its mother in a confined ill-ventilated traditional house called Ume Kbubu. The two are exposed to fumes from a wood-burning stove in the belief that it will improve the general health and well-being of the new mother and her baby. Behavioral intervention of reducing this period of confinement from 40 days to 4 days significantly reduced the effects of the wood-burning stove. Similarly, behavioral interventions can be implemented to avoid the build-up of particulate matter from the burning of incense sticks. Advocating the use of mosquito bed-nets and window nets instead of the burning of mosquito coils or using aerosol mosquito repellents can assist in reducing the burden of household air pollutants.

It is not sufficient to improve only ventilation in households. Highly energy-efficient homes have also been found to be associated with increased risks of developing asthma ⁷¹ . This is attributed to increased dampness within such high energy-efficient houses. It is therefore important not just to ensure energy efficiency and changed building materials but also to provide a dry and warm environment with good ventilation to minimize indoor dampness.

Air purifiers and ionizers have also been suggested as remedial measures to curb indoor air pollution. The use of air purifiers for just a couple of hours has been shown to reduce PM _2.5 concentrations by 57% and caused a reduction in serum levels of inflammatory markers, indicating good cardiovascular outcomes ^{72, 73} . However, no such statistical improvements have been shown in respiratory outcomes.

Using indoor plants, wet-mopping floors, avoiding the use of heavy upholstery, and refraining from smoking within confined home environments can further facilitate a reduction in household air pollutants. There are several other remedial measures suggested to reduce the health effects of household air pollution. However, these are beyond the scope of this review.