Overall exposure
5.4 Indoor Air Pollution
5.4.1 Pollutants and their sources
Many air pollutants are present in indoor en-vironments, and their concentrations may vary widely not only between locations but also between buildings at the same location and even rooms in the same building. As shown in Table 5.7, the sources of airborne contaminants in indoor environments are numerous [57–59]. Some indoor air pollu-tants are primarily generated outdoors, but may also have indoor sources. Such pollu-tants include SPM, SO2, NO2, carbon mon-oxide (CO), photochemical oxidants, lead and some VOC. In the absence of additional indoor sources, the ratio of indoor to out-door concentrations is, in general, in the range of 0.7 to 1.3.
The pollutants generated or released in-doors can be divided into two categories.
The first is related to human presence or ac-tivities. For example, combustion, particu-larly with inadequate ventilation, or the evap-oration of solvents can result in substantial releases of gaseous and particulate pollu-tants including CO, carbon dioxide (CO2), NO2, SO2, water vapour and VOC. The prin-cipal combustion sources are tobacco smok-ing, gas cooking stoves and unvented heaters (such as kerosene heaters), as well as the burning of wood or coal in stoves and open fireplaces. The second category of pollutants with predominantly indoor sources are those released from buildings and furnishings. For-maldehyde may be released from a number of building materials, particularly from par-ticle board and plywood as well as from urea formaldehyde foam insulation, furnishings and/or household products. Other VOC may also be released from furnishings or house-hold products. Asbestos and other mineral fibres used in insulation may be present in indoor air. Radon and its decay products may accumulate indoors, mainly from the ground beneath a given building but some-times from the building materials them-selves.
In addition to chemical and physical con-taminants, various biological agents that may affect human health are frequently
pre-sent in indoor air environments [7]. Bacteria (including actinomycetes), fungal spores, algae, the fragments and droppings of house-dust mites, and animal dander are among the most prevalent. These produce illness through infection of the respiratory tract or by stimulating an immune response. Many indoor environments provide sufficient moisture and an appropriate temperature for the growth of microorganisms and mites (see also Chapter 14). Measures taken to conserve energy may enhance these condi-tions. For example, a recent study from Sweden found such homes to have a level of infestation with domestic mites similar to that reported from subtropical regions [60].
5.4.2 Effects on health
The large diversity of indoor air pollutants may have various adverse effects on health, ranging from sensory annoyance or
discom-fort to severe effects [58,59,61,62]. Estab-lishing causal relationships between specific exposures and defined health outcomes is often difficult, however, mainly because of the multifactorial nature of exposure to in-door air pollutants. Few studies have been carried out in Europe to quantify particular exposures and related health effects. Table 5.8 lists examples of health effects that may result from exposure to certain indoor air pollutants.
Indoor air pollutants may affect different organs and systems but the primary target of most air pollutants is the respiratory system.
Observed effects include changes in pulmon-ary function, an increase in respiratory symptoms (such as coughing, wheezing, shortness of breath and phlegm), respiratory infections, the sensitization of airways to al-lergens in the indoor environment, and asth-matic attacks. The principal agents affecting the respiratory system include combustion products (such as particulate matter, NO2 and SO2), environmental tobacco smoke,
Table 5.7: Major indoor air pollutants and their principal sources
162 Air Pollution
formaldehyde, allergens and infectious or-ganisms.
Among the effects of indoor air pollutants on the respiratory system, those of environ-mental tobacco smoke are relatively well es-tablished. A recent review from the United States showed that the incidence of acute re-spiratory infections in children of mothers smoking at home is 1.5–2 times higher than that in other children [63]. The correlation depends on the extent of the exposure and is more pronounced in younger children, as in-dicated by recent epidemiological studies. In a cohort of British children, the prevalence of wheezy bronchitis in those aged 1–
10 years was 14 % higher when mothers smoked 4–13 cigarettes a day, and 49 % higher when mothers smoked more, than in the children of nonsmoking mothers [64].
The effects observed in infants are even more pronounced. In a large cohort followed through the first year of life, the prevalence of lower respiratory illness was 50 % higher when the mother smoked, and the risk in-creased to 2.7 when the mother smoked at least a pack of cigarettes per day and the child stayed at home rather than attending daycare [65].
Other effects on the respiratory system may be related to an increased indoor expo-sure to NO2 associated with the use of gas
stoves. Analysis of data from 11 studies [66]
showed about a 20 % increase in the inci-dence of respiratory illness in children with a 30 µg/m3 increase in long-term average con-centrations of NO2. In addition, respiratory function in children, especially in asth-matics, was found to be adversely affected by increased levels of formaldehyde emitted by building materials and home equipment [67].
Indoor air pollutants may affect the im-mune system and provoke allergic responses, including rhinitis and conjunctivitis, asthma and alveolitis, and atopic dermatitis. The continued exposure of sensitized individuals may result in permanent lung damage. Pollu-tants triggering such responses include al-lergens from house-dust mites, dander from pets, fungi (including yeasts and moulds) and bacteria (including actinomycetes) [68].
Epidemiological studies have shown that ex-posure to house-dust mites during childhood may be an important risk factor for the devel-opment of allergic asthma [69].
Some indoor air pollutants are carcino-genic. In particular, environmental tobacco smoke (which contains a number of carcino-genic agents), radon and its decay products, and asbestos fibres have been demonstrated to increase the risk of lung cancer develop-ing, and there may be synergism between the
Table 5.8: Health effects of selected indoor air pollutants
actions of these agents. Chapter 12 discusses in detail the cancer risk related to indoor radon exposure. Several epidemiological studies have examined the relationship be-tween lung cancer and exposure to environ-mental tobacco smoke. The combined evi-dence from 25 of them indicates an increase in risk by 20–30 % in nonsmokers married to smokers [70]. For other potential carci-nogens – including benzene, formaldehyde, polynuclear aromatic hydrocarbons (PAHs), pesticides and nitrosamines – no firm evi-dence exists of carcinogenic effects at the concentrations encountered in non-occupa-tional indoor environments. Indoor environ-ments, however, may have a significant im-pact on the extent of total exposure to these substances from air. As illustrated by an American study [71], the contribution to total individual exposure to benzene in air from active and passive smoking and other personal activities at home may amount to 78 %, with an additional 5 % due to exposure inside vehicles during travel. These indoor sources are only responsible for up to 3 % of all benzene emissions to air, while motor ve-hicles and industry contribute 83 % and 14 %, respectively.
Indoor air pollutants may affect the skin and mucosal tissues, resulting either in pri-mary sensory irritation or in irritation sec-ondary to inflammatory changes in the skin, mucous membranes or other tissues. In gen-eral, the symptoms observed are non-spe-cific. Formaldehyde and other aldehydes, VOC and environmental tobacco smoke may evoke such responses.
The effects of indoor air pollutants on the nervous system encompass effects on the senses and the central nervous system. Sen-sory effects include those related to odours and eye irritation or dry skin, which may be caused by VOC, formaldehyde and environ-mental tobacco smoke. Effects on the cen-tral nervous system include toxic and hy-poxic or anoxic damage to nerve cells. Such effects may be the consequence of exposure to VOC, various pesticides and CO.
Exposure to CO may also adversely affect the cardiovascular system; cardiac
arrhyth-mia and aggravation of angina symptoms have been observed at the relatively low ex-posure levels encountered indoors or in en-closed spaces such as inside vehicles or in street tunnels [72]. Despite the recognition of the hazard of CO poisoning, domestic fatalities from acute exposure are still re-ported, often as a result of faulty installation or use of gas appliances [73].
The sick building syndrome is a term used to describe a set of various symptoms that are observed mainly, but not exclusively, in air-conditioned buildings not directly con-taminated by industrial processes. They typi-cally include mucous membrane and eye irri-tation, cough, chest tightness, fatigue, head-ache and malaise.
The following criteria are used to define the sick building syndrome [62]. First, a high proportion of the occupants of the building react. Second, the symptoms and reactions observed include acute physiologi-cal or sensory reactions (such as sensory irri-tation of mucous membranes or skin, gen-eral malaise or headache, non-specific hyper-sensitivity reactions, dryness of the skin, and complaints about odours or taste) and psy-chosocial reactions (such as decreased pro-ductivity, contact with primary health care services, and initiatives to modify the indoor environment). Irritation in eyes, nose and throat dominate, and systemic symptoms are infrequent. Finally, there is no obvious causal relationship with high exposure to a single agent.
In principle, the symptoms are mainly re-ports of discomfort or the feeling of being less than well. In general, these symptoms disappear when the person leaves the build-ing. The sick building syndrome usually can-not be attributed to excessive exposure to a known contaminant or to a defective venti-lation system. A number of factors may be involved:
• physical factors, including temperature, relative humidity, ventilation rate, artifi-cial light, noise and vibration;
• chemical factors, including environmental tobacco smoke, formaldehyde, VOC,
pes-164 Air Pollution
ticides, odorous compounds, CO, CO2, NO2, and O3; and
• biological and psychological factors.
The interaction of several factors involving different reaction mechanisms is assumed to cause the syndrome, but there is as yet no clear evidence of any exposure–effect rela-tionship [74].
5.4.3 Exposure in Europe
Indoor concentrations of air pollutants are influenced by outdoor levels, indoor sources, the rate of exchange between indoor and out-door air, and the characteristics and furnish-ings of buildfurnish-ings. Concentrations are subject to geographical, seasonal and diurnal vari-ations. The exposure varies significantly be-tween and within various populations, de-pending on occupancy patterns and the fac-tors already mentioned.
Indoor levels of NO2, for example, depend on the presence of gas heaters and cooking ranges (used in 20–80 % of houses in some countries). In dwellings with gas equipment, the average NO2 concentrations (over 2–7 days) observed in European countries were 20–40 µg/m3 in living rooms and 40–70 µg/
m3 in kitchens, while they were 10–20 µg/m3 in dwellings without gas equipment. These values may be doubled in rooms facing streets with heavy motor traffic. The average levels reported by five countries fell within these ranges. These exposure levels may have an effect on respiratory function, as already discussed. People may be exposed to higher NO2 levels under certain circumstances, as in dwellings equipped with unvented cooking ranges. In addition, short-term measure-ments reveal concentrations that may be fivefold higher than those averaged over sev-eral days. Peak values of up to 3800 µg/m3 for 1 minute have been measured in the Ne-therlands in kitchens with unvented gas cooking ranges [75,76].
In general, average CO concentrations in Europe are well below the short-term WHO guideline values of 60 mg/m3 for 30 minutes or 30 mg/m3 for 1 hour. In kitchens with gas
stoves, short-term values of up to 15 mg/m3 have been measured. The highest values were measured in bars and pubs, where smoking is common, with average concen-trations of 10–20 mg/m3 but peak levels of up to 30 mg/m3 [76].
Formaldehyde concentrations in indoor air, as reported by five countries, ranged from 9 to 70 µg/m3. These values agree well with the data reviewed in the literature, where mean indoor air levels of formalde-hyde were found to be 5–63 µg/m3 with higher values occasionally encountered, es-pecially in dwellings with urea formaldehyde foam insulation [77]. The WHO guideline value of 100 µg/m3 for short-term exposure, which is based on irritative effects, is on average not exceeded.
The geographical variation in exposure to radon in the European Region is discussed in Chapter 12. In general, average levels in-doors are 20–70 Bq/m3 [78] although they may be ten times higher in certain areas.
Exposure to environmental tobacco smoke is an important factor in indoor air quality assessment. The particulate and va-pour phases of such smoke are complex mix-tures of several thousand chemicals, includ-ing known carcinogens such as nitrosamines and benzene. One of the most commonly used indicators of concentrations of environ-mental tobacco smoke is the level of PM10 pollution. This is 2–3 times higher in houses with smokers than in other houses [35]. Nic-otine is present in the vapour phase, with concentrations of up to 10 µg/m3 in houses with smokers. Data from nine countries re-vealed that 33–66 % of households had at least one smoker. The proportion of children with mothers smoking at home varied from 20 % to 50 %, that of children with fathers smoking at home from 41 % to 57 %. A recent German study confirmed these estimates, in-dicating that exposure to maternal and/or paternal smoking was considerably higher among children of less educated parents [79]. Exposure to tobacco smoke, particu-larly the exposure of children, is therefore a major environmental health problem throughout the European Region.
Scattered data are available on exposure to other indoor air pollutants, including VOC and biological contaminants, but as-sessments are hindered by large differences in sampling design and methodology, as well as in analytical techniques. A comprehensive study conducted in 479 dwellings in Ger-many measured a total of 172 different VOC [80]. Mean concentrations were almost al-ways below the WHO guideline values. The full extent of European population exposure to biological particles promoting allergic sen-sitization is difficult to assess. Some evi-dence links dampness in houses with mite numbers, mould growth and respiratory ef-fects. Surveys from the Netherlands and the United Kingdom suggest that around 15–
20 % of houses may be affected to some ex-tent by dampness [68].
The potentially important contribution of indoor air pollutants to total exposure requires the indoor environment to be con-sidered in the overall assessment of the health risks of air pollution. There is a strong need for coordinated studies of exposure to indoor air pollutants and of their possible health effects.
The chemical composition of the global tro-posphere, the lowest 10–15 km of the atmos-phere, is changing. Various trace gases that are chemically inert in the troposphere (greenhouse gases), such as CO2, nitrous oxide (N2O) and chlorofluorocarbons (CFCs), as well as some more reactive com-ponents such as methane (CH4), CO and O3, show a strong increase in concentration.
This is associated with the possibility of glo-bal warming.
The observed depletion of the ozone layer in the stratosphere (the upper atmosphere, above 10–15 km) during recent decades is a truly global air pollution problem. It is now