Environmental Health: Air Pollution and Quality -- Indoor / Outdoor

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The purpose of this section is to look at the constituents of air, the pollutants it carries, and other relevant aspects that have a bearing on indoor air quality, including the positive measures we can take to improve air quality in our homes.

Air and water are intertwined in their role as the lifeblood of the planet, being the prime agents of cycling and recycling the elements of life. There are an enormous number of different elements and compounds that are cycled through the biosphere within this circulation system. Many of these cycles are only just beginning to be understood. We humans are currently using the earth’s circulatory system for our own waste-disposal purposes, blithely assuming that nature will recycle and absorb anything we throw into it. On a global scale we are beginning to see just how false this assumption is while on a domestic scale we have problems of an altogether different nature.

Quite sensibly, we are reacting to the need to conserve energy—in part to control global warming—by draft-proofing our homes. Without adequate ventilation, though, we are in danger of concentrating our own home-produced pollutants to the point where our health may be threatened. Internal air quality is undoubtedly one of the key factors in the make up of a healthy indoor environment. Most of us spend an average of 90% of our lives indoors, the highest proportion of this being in winter, when indoor air pollution is at its worst. We are thus much more likely to breathe in and absorb into our bodies any gas, vapor, or airborne particle that escapes into the air indoors.


The following gases are all constituents of air, and have been singled out for their relevance to indoor air quality. Their normal atmospheric percentage by volume is given in brackets, though indoors the relative proportions may vary considerably).

Nitrogen (N)

Nitrogen [78%] the most abundant gas, plays an important part both as a sustainer of atmospheric pressure and as a natural dilutant of oxygen (see below). Nitrogen is also an essential element for life: it is contained in proteins and DNA, and is extracted from the air by nitrogen-fixing microorganisms in the soil.

Oxygen (O2)

Oxygen [21%] is the second most abundant component of air, and is both the most vital and one of the most destructive elements in the atmosphere. It is highly reactive: evidence of this is seen every time we witness a fire. It is likely that fire itself is the reason for the percentage of oxygen in the atmosphere remaining stable at 21%. If it is higher than this, the likelihood of spontaneous combustion climbs rapidly (a 1% rise doubles the likelihood of combustion). At a level lower than 15%, however, combustion does not take place. Oxygen also performs metabolic combustion in our muscles, providing motive power for our bodies. As an oxidant it kills bacteria and thus acts as an antiseptic. However, its toxicity also has an impact on the aging process and maybe a major factor in determining our life span.

Carbon Dioxide (CO2)

Carbon dioxide [0.032%] is a key metabolic gas, influencing climate, plant growth, and oxygen production. For such an important gas it is surprising that it comprises such a tiny proportion of air! In terms of its role inside the house, CO2 usually occurs at slightly higher concentrations than outside as a result of exhalation by inhabitants and flueless combustion. Running flueless equipment in poorly ventilated space leads to the buildup of the products of combustion. CO2 although the most abundant product of combustion, is the least harmful of the gases produced. In high concentrations it leads to a feeling of drowsiness and stuffiness. The other main products of combustion—carbon monoxide, nitrogen dioxide, and sulfur dioxide—are much more dangerous.

Methane (CH4)

Methane [0.00015%] is the major constituent of natural gas; its role as an indoor air contaminant is in most cases negligible. The exception is where methane seeps up from the ground or from leaks in gas pipes, since it can build up in unventilated spaces and cause an explosion.

Carbon Monoxide (CO)

Carbon monoxide is one of the more important indoor contaminants because of its extreme toxicity. It is produced by incomplete combustion. Within buildings it may result from tobacco smoke or flueless cooking or heating appliances. Breathing in CO causes a condition known as hypoxia: the combining of CO with hemoglobin in the blood, cutting off its ability to carry oxygen, which results in tissue damage and even death. Carbon monoxide used to be a constituent of town-supplied gas before natural gas. Indoor concentrations rarely reach serious levels; however, injury and sometimes death has resulted from the backflow of combustion products from incorrectly installed water heaters.

Sulfur Dioxide (SO2)

Sulfur dioxide, a pungent gas and one of the gases of combustion produced from burning coal, wood, oil, or paraffin, was once responsible for urban smog and is now the main cause of acid rain. It is rarely a problem now inside the home, except when produced by combustion with inadequate ventilation.

Hydrogen Sulfide (H2S)

Hydrogen sulfide is a very poisonous and very smelly gas (rotten eggs) produced by protein decomposition and by burning sulfurous coal. You would normally be repulsed by its smell long before it became dangerous.

Ammonia (NH3)

Ammonia is an important gas in terms of ecological cycles. Gleaning fluids are the most common source for indoor air pollution. As it has a strong smell, it is possible to avoid dangerous levels if you remain aware that strong, unpleasant smells are almost certainly unhealthy for you.

Environmental Tobacco Smoke

Environmental Tobacco Smoke (ETS) is a mixture of exhaled mainstream smoke (MS) and the side-stream smoke (SS) which evolves between puffs. ETS is a mixture of several thousand constituents, some of the more commonly measured of which are particulates, nicotine, carbon monoxide, benzene, nitrogen dioxide, acrolein, and formaldehyde. Besides the objectionable smell, high concentrations cause irritation to the eyes, nose, and throat, as well as a reduction in visibility. Irritation can be further exacerbated by low humidities. Long-term effects are an increased incidence of respiratory disease among children of smoking parents and an increased risk of lung cancer. The dangers of passive (or “‘secondhand”) smoking are at last being realized, and the banning of smoking in public places is gaining wide acceptance. Smoking is also an issue each household needs to confront.

Organic Compounds

As regards the air quality in buildings, a growing problem has been the increase in the number of volatile organic vapors. Some examples of these, along with their sources, are given below: little is known of the effects of exposure to low concentrations of many of these substances, particularly the effects of exposure to various mixtures of them. However, if you have a general susceptibility to indoor pollution, it would be wise to reduce your exposure to these chemicals to a minimum.

Volatile Organic Vapors

Commonly Found In:

Ethyl benzene

Methylene chloride





Carbon tetrachloride Chlorobenzene





Ethyl acetate

Nail varnish, tobacco smoke Adhesives, spot cleaners, paint remover, particleboard

Caulking, particleboard, floor coverings, fiberboard, tobacco smoke Grease cleaners, dry-cleaning fluid Paint solvents, DDT, phenol (carbolic acid)

Adhesives, sealing tape, some wallpaper, paint, plastic floor covering Cleaning fluids, correction fluids Paint solvents

Adhesives, wallpaper, flooring Linoleum, varnishes, perfumes, artificial leather

Floor and wall coverings

Paint removers, aerosol finishers

Insulation foam, fiberboard, tobacco



This is a vapor given off from urea-formaldehyde glues, foams, and certain plastics. As a result it “outgases” from particleboards that use formaldehyde glues, such as chipboard and insulating foams. This can be a particular problem where new building work has been carried out or new chip- board-based fittings or furniture has been installed. This vapor could build up in houses where there is poor ventilation.

Wood Preservatives

These chemicals are a mixture of solvents and pesticides such as pentachlorophenol, gamma-HCH or Lindane, dieldrin, and tributyltinoxide (TBTO). They are extremely dangerous. Exposure to the active ingredients in treated buildings is usually through the inhalation of dust particles which have the compound attached to them. As well as being irritants and nerve poisons, some of these compounds target specific organs, such as the liver, and can accumulate there from a number of sources, including through the food chain from agricultural use. Dieldrin is the most dangerous in the longer term: for householders the greatest risk is from DIY work performed without safety clothing and adequate protection. Newly treated lumber is a major source of toxins and should be left well ventilated for as long as possible before use.

Asbestos Fibers

The name asbestos is given to a number of fibrous minerals that have all been used in the past for either insulation, fire-proofing, or reinforcement. Blue asbestos is known to be particularly carcinogenic (when fine particles are taken into the lungs). As a result of wide publicity, the dangers of this material are well known. In terms of existing housing, the main threat is from loosely applied insulation: this should be identified and safely removed by specialists. Due to the risk of fire releasing particles into a built-up environment, asbestos in other, seemingly inaccessible, locations should also be removed as soon as the opportunity arises.

Airborne Microorganisms

Bacteria, mold spores, and amoebas are all commonly found in indoor air. If they find a suitable place to settle, where the temperature, humidity, and nutrients are favorable, they may form colonies, multiply, become airborne, and enter the respiratory systems of occupants. Prolonged or heavy contamination can cause an allergic reaction in the nasal mucus membrane or the lungs.

Airborne House Dust

Apart from the above microorganisms, house dust contains, among other things: hairs and fibers, dandruff, particles of wood, plastic, etc.; airborne artic1es from outside; soil particles from shoes; and the droppings from dust mites. These will all become airborne if small enough and stirred up by moving furnishings, cleaning, and vacuuming. Dust mite droppings in particular can pass through a vacuum filter and cause allergic reactions in some people. You can, however, buy special vacuum cleaners with filters which do not allow mites and other irritants through.

Apart from reducing the number of furnishings that carry dust mites, one way of expelling very fine particles to the outside is to use a centralized vacuum system installed on an external wall. Alternatively, there are vacuum cleaners that use water as a filter.


Radon, a highly dangerous radioactive gas, is mainly dealt with in the RADI ATION section. The toxicity of radon comes from alpha radiation, which is particularly dangerous to lung tissue.


Apart from the natural constituents of air and the commonly found indoor pollutants, there are a number of other important characteristics of air quality:

Relative Humidity

Water vapor is an important variable constituent of air. It plays a key role in controlling temperature and weather on the planet. Inside our homes it has an important effect on our health and comfort. The amount of water vapor in the air is expressed in terms of relative humidity, with 100% denoting totally saturated air and 0% totally dry air. The amount of water vapor that can be held in the air increases with temperature, so that very warm air can carry several times as much water as very cold air and still register the same relative humidity level. Very dry air when it is warm causes our mucus membranes to dry out, leading to increased vulnerability to infections, whereas very humid air can cause bronchial problems.

There is a tendency for air to become too dry in winter; fortunately, a number of methods can be used to increase moisture levels. There are various types of humidifiers on the market, ranging from simple trays for water that attach to a radiator to electric appliances that send a fine mist of water into the air. Alternatively, you can use houseplants or change your clothes-drying arrangements, hanging damp clothes on wooden racks to dry and , at the same time, humidify a room. For health and comfort it is generally advisable to keep humidity levels above 20% to 30%. In order to monitor these levels it is well worth acquiring a small number of hygrometers to place in key locations in your home, such as your kitchen, living room, or bedroom.

In certain circumstances humidity levels can become too high. This can be caused by lack of adequate ventilation, with moisture from warm kitchens and bathrooms dispersing to other, cooler parts of the house. In basements, high relative humidity can be caused by damp penetration. Above 70% humidity, mold growth is encouraged, and mold spores can cause allergic reactions. In extreme cases a dehumidifier can be used, but in most cases increasing ventilation rates and attending to the source of the humidity in the first place are the most appropriate solutions. In kitchens and bathrooms, when humidity levels become too high, it is worth considering using a humidistat attached to mechanical ventilation to keep the humidity to preset levels.


Ions are positively and negatively charged molecules. Outdoors, in unpolluted places, the air contains some 16,000 to 32,000 ions per cubic inch, in a ratio of five positive to four negative. This proportion gives a feeling of general well-being. In certain conditions, for instance before thunder storms, the negative ions lose their charge and a surfeit of positive ions is produced, causing unpleasant feelings of tension, irritability, depression, and even physical disorders.

Inside the home, negative ions are depleted by electrical fields from TVs and other electrical appliances, static from synthetic fibers (most commonly from carpets), and dry air. Ionizers are cheap units which replace these negatively charged ions; many people find them helpful. Ionizers also have the effect of cleaning the air, as the ions become attached to dust particles which in turn are attracted to surfaces in the surroundings. If you want to use an ionizer and wish to avoid this accumulation of fine dust, it would be worth acquiring one with a filter.

Odor and Our Sense of Smell

Our noses are extremely sensitive organs: our sense of smell is thousands of times more acute than our sense of taste. Air pollution can dull and even damage our sense of smell. An unpleasant smell can be a warning that the air could be unhealthy. In our daily lives, however, we often feel that we have little choice and are persuaded to put up with unpleasant smells. This can cause psychological stress, apart from any possible toxic effects.

We should be able to control the odors in our homes; there should be no need to put up with unnecessary unpleasant smells. We can remove the sources of unpleasant smells and begin to create a more harmonious environment by using natural materials. We can also incorporate pleasant odors from sweet-smelling plants.

Ventilation Rates

Ventilation is the means by which stale air is exchanged for fresh air from outside. The important measure for this is the number of times that air is exchanged, simply referred to as air changes per hour. The need for ventilation is dependent on a number of factors, which include the number of people using the room, the overall volume, the humidity, the temperature, any sources of combustion, the odor level, any sources of toxicity, and , finally, whether anyone is smoking. The practical aspects of how to develop a ventilation strategy are examined in the DRAFT-PROOFING and VENTILATION section.

Use of Plants

If you are fond of plants, you have the opportunity to use their many properties to improve air quality. They can be used as humidifiers, as natural filters to absorb toxins such as formaldehyde, and to add fragrance (see PLANTS section).


+ Identify and reduce the sources of contamination. Eliminate the worst offenders, such as open sources of combustion, chemicals, and dust.

+ Increase natural ventilation and work on finding the right balance between air changes and energy conservation (see the DRAFT-PROOFING and VENTILATION section).

+ Use mechanical ventilation if necessary to exhaust water vapor, fumes, or the products of combustion at their source. Maintain flues or filters, and repair defective heating appliances.

+ Use plants to help clean the air, moderate humidity levels, and introduce fragrance (see PLANTS section).

+ When humidity levels are too low, use humidifiers, plants, or even clothes-drying, to increase moisture. When the humidity is too high, try to reduce sources of moisture. Increase natural ventilation, attach a humidistat to mechanical ventilation, or, in extreme cases, use a dehumidifier.

+ Conserve negative ions by avoiding a dry atmosphere, and synthetic fabrics and carpets. Consider using an ionizer.

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