It refers to the physical, chemical, and biological characteristics of air in the indoor environment within a home, building, or an institution or commercial facility. Indoor air pollution is a concern in the developed countries, where energy efficiency improvements sometimes make houses relatively airtight, reducing ventilation and raising pollutant levels. Indoor air problems can be subtle and do not always produce easily recognized impacts on health. Different conditions are responsible for indoor air pollution in the rural areas and the urban areas.

On average, 90 out of 100 children are affected by indoor air pollution every hour in Bangladesh. People living in urban slums, high-rise apartments or in old and dark houses suffer the most. The indoor air pollutants infect almost 88% of the total population. More than 60% of the total child population (under 5) is living with high death or disability threat in Bangladesh because of indoor pollution.

A study done by Environment and Social Development Organization-ESDO mentioned that in Bangladesh, both urban & rural areas face the greatest threat from indoor pollution, where about 110 million (11cror) people continue to rely on traditional fuels such as firewood, charcoal, and cow dung, LPG or natural gas and crud oil for cooking and heating. Concentrations of indoor pollutants in households that burn traditional fuels are alarming. Burning such fuels produces large amount of smoke and other air pollutants in the confined space of the home, resulting in high exposure. Women and children are the groups most vulnerable as they spend more time indoors and are exposed to the smoke. People spend their day in different places in the kitchen, indoors at home and at work, outdoors in fields and crowded roads. Because these different places can have very different levels of air pollution, how much pollution a person breathes in depends very much on who he or she is rich or poor, man or woman, infant or elderly. The study indicate that particulate matter concentrations in kitchens due to the burning of bio-fuels are known to be as higher as 30-55 times the WHO standard, while concentrations at the workplace for primary workers can be as high as 15-25 times the standard, though outdoor concentrations are about 2.5-3.5 times the WHO standard in urban and rural Bangladesh, and 10-15 times the standard in slums in Bangladesh. Air pollution causes a range of health impacts from increasing the risk of death to increased asthma attacks, causing losses of income and greater medical expenses. The study estimates that nationwide, the deaths and asthma attacks due to various forms of indoor pollution are three times that due to outdoor pollution. Though most research efforts and media attention has focused on outdoors air pollution but ignoring the indoor air situation, which is most important for human life.

The study in urban and rural Bangladesh estimated the amount of time women actually spent near cook stoves and found that about 80-100% of daily exposure of women and children was contributed by indoor pollution. ESDO‘s study targeted infants living in the Dhaka slums, and studied their health status, too.

In 1992, the World Bank designated indoor air pollution in the developing countries as one of the four most critical global environmental problems. Daily averages of pollutant level emitted indoors often exceed current WHO guidelines and acceptable levels. Although many hundreds of separate chemical agents have been identified in the smoke from bio-fuels, the four most serious pollutants are particulates, carbon monoxide, polycyclic organic matter, and formaldehyde. Unfortunately, little monitoring has been done in rural and poor urban indoor environments in a manner that is statistically rigorous.

According to the study of the ESDO and EPA of US, in urban areas, exposure to indoor air pollution has increased due to a variety of reasons, including the construction of more tightly sealed buildings, reduced ventilation, the use of synthetic materials for building and furnishing and the use of chemical products, pesticides, and household care products. Indoor air pollution can begin within the building or be drawn in from outdoors. Other than nitrogen dioxide, carbon monoxide, and lead, there are a number of other pollutants that affect the air quality in an enclosed space.

The EPA has found that the air in some office buildings is 100 times more polluted than the air outside. Poor ventilation causes about half of the indoor air pollution problems. The rest come from specific sources such as copying machines, electrical and telephone cables, mold and microbe-harboring air conditioning systems and ducts, cleaning fluids, cigarette smoke, carpet, latex caulk and paint, vinyl molding, linoleum tile, and building materials and furniture that emit air pollutants such as formaldehyde. A major indoor pollutant is radon-222, a colorless, odorless, tasteless, naturally occurring radioactive gas produced by the radioactive decay of uranium-238. "According to studies by the EPA and the National Research Council of US, exposure to radon is second only to smoking as a cause of lung cancer.

About 2.8 million premature deaths occur every year due to indoor air pollution. As many as 80% of all diseases in the world are associated with water usage or poor environmental hygiene. Though women are considered as "the key" to ensuring food security at the household level as food producers, providers, and contributors, they inordinately suffer from nutritional deficiency disorders due to a gender bias. The household environment of the poor people, especially women and children in developing countries, carries the biggest risks to health. These claims are made by a new analysis of the health impacts of the domestic environment on women and children, jointly carried out by the United Nations Environment Programme (UNEP) & EPA.

According to the ESDO and EPA study, the major organic compound and gases emit in indoor are, Volatile organic compounds originate mainly from solvents and chemicals. The main indoor sources are perfumes, hair sprays, furniture polish, glues, air fresheners, moth repellents, wood preservatives, and many other products used in the house. The main health effect is the imitation of the eye, nose and throat. In more severe cases there may be headaches, nausea and loss of coordination. In the long term, some of the pollutants are suspected to damage to the liver and other parts of the body.

Tobacco smoke generates a wide range of harmful chemicals and is known to cause cancer. It is well known that passive smoking causes a wide range of problems to the passive smoker (the person who is in the same room with a smoker and is not himself/herself a smoker) ranging from burning eyes, nose, and throat irritation to cancer, bronchitis, severe asthma, and a decrease in lung function.

Pesticides if used carefully and the manufacturers, instructions followed carefully they do not cause too much harm to the indoor air. Biological pollutants include pollen from plants, mite, hair from pets, fungi, parasites, and some bacteria. Most of them are allergens and can cause asthma, hay fever, and other allergic diseases. Formaldehyde is a gas that comes mainly from carpets, particleboards, and insulation foam. It causes irritation to the eyes and nose and may cause allergies in some people. Asbestos is mainly a concern because it is suspected to cause cancer.

Radon is a gas that is emitted naturally by the soil. Due to modern houses having poor ventilation, it is confined inside the house causing harm to the dwellers

By the concern of WHO, Cause of Indoor Pollution Ranks of Disease are;

1. Asthma
2. Lower respiratory infections
3. Chronic bronchitis
4. Chronic obstructive airway disease
5. Lung cancer
6. Diarrheal diseases
7. Conditions arising during the prenatal period
8. Unipolar major depression
9. Ischemic heart disease
10. Adverse reproductive outcome