This literature review on the health effects of chemicals in the indoor environment reflects that most of the studies so far have investigated the development and exacerbation of asthma, respiratory symptoms, allergic symptoms, and symptoms included in the "sick building syndrome” (SBS). Few studies exist that have studied endocrine disruption or cancer development in relation to exposure to chemicals in the indoor environment.
There is an extensive body of literature estimating the exposure by measuring chemicals and their metabolites in body fluids, mainly in blood or urine, and relating this exposure to a variety of health effects. There might, however, be different sources of this internal exposure, and these sources have not been examined in most of these studies. Such studies have been included in this review only if it is well known that materials in the indoor environment are an important source of exposure.
Several aldehydes can be measured indoors, including acetaldehyde, formaldehyde, propionaldehyde, butyraldehyde, pentanal, hexanal, and benzaldehyde. Formaldehyde is common indoors and can be emitted from many construction materials, furniture, etc. In general, formaldehyde levels are highest in newly built or renovated buildings or where there is new furniture containing formaldehyde-based resins. The formaldehyde level in homes is usually 20–30 µg/m³ in Sweden, but levels may vary. Formaldehyde is one of the most-studied indoor air pollutants in indoor epidemiology, and epidemiological studies and meta-analyses confirm the importance of formaldehyde in the development of asthma and other allergic symptoms as well as SBS symptoms in both children and adults and in homes and schools. The WHO guideline value for formaldehyde in the indoor environment is a mean of 100 µg/m³ over a period of 30 minutes. The critical effect is sensory irritation. Acetaldehyde can be emitted from building materials such as wood, cork, and linoleum. Commonly, the indoor level of acetaldehyde in Sweden is about 10 µg/m³, and the odour threshold is estimated to be 25 µg/m³. Several studies have found associations between air concentrations of acetaldehyde in schools or homes and health effects such as wheeze, asthma, and SBS symptoms. Acrolein is a reactive aldehyde formed during the combustion of wood, plastic, diesel, and paraffin wax and from smoking and cooking. Some studies have found associations between levels of acrolein in indoor environments and health effects such as eye irritation and asthma, but there is a lack of information on indoor concentrations of acrolein in Sweden.
Indoor levels of nitrogen dioxide (NO2) in Swedish homes, schools, and preschools are usually low and are primarily affected by proximity to vehicle traffic. The indoor concentration of NO2 in Sweden is typically below 20 µg/m3 but can be higher in homes with gas stoves. The indoor levels of NO2 in homes with gas stoves have been shown to be associated with health effects such as wheeze, shortness of breath, asthma symptoms, and reduced lung function. The WHO guideline values for NO2 in indoor environments are 40 µg/m³ (annual mean) and 200 µg/m³ (one hour mean). The critical health effect is airway inflammation and airway symptoms.
Sources of BTEX (benzene, toluene, ethylbenzene, xylene) in indoor environments include infiltration of outdoor air pollution, tobacco smoke, paint, glue, and some building materials. Studies have demonstrated associations between indoor concentrations of BTEX and wheeze, asthma, and SBS symptoms. Benzene is classified as a carcinogenic compound, and the WHO does not report any guideline value for benzene. In Sweden, the population's average exposure to benzene is estimated to be 2 µg/m³.
Terpenes are commonly found in indoor environments in Sweden because there is extensive use of coniferous wood in buildings, furniture, and furnishings. Some studies have reported associations between the concentrations of a-pinene and limonene in homes and SBS symptoms and bronchial hyperreactivity, but there are relatively few epidemiological studies on indoor terpenes. Terpenes can be oxidized to form ultrafine particles and other compounds, but there are few studies on the potential health effects of these reaction products in indoor environments.
PDCB (p-dichlorobenzene, also known as 1,4-dichlorobenzene) is used in insect repellents and "air fresheners" for toilets, etc. The average indoor concentration of PDCB in homes in Sweden is around 1 µg/m3. Pentachlorophenol (PCP) and its sodium salt have been widely used, mainly as wood preservatives. In Sweden PCP is no longer used, but the chemical can still be found in many older buildings. Under the influence of moisture, PCP can form chloroanisoles, which can produce indoor odours similar to mould odour. The International Agency for Research on Cancer (IARC) has classified PDCB and PCP as possibly carcinogenic to humans (Group 2B).
The compound 2-ethyl-1-hexanol (2E1H) is a higher alcohol that can be emitted from plastic materials, e.g. PVC flooring. Studies have shown associations between 2E1H and SBS and asthma symptoms. Texanol (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate) is often used in water-based paints. TXIB (2,2,4-trimethyl-1,3-pentanediol diisobutyrate) is used as a plasticizer in PVC and polyurethane materials, wallpaper, and leather goods. TXIB can contribute to odours indoors and can cause irritation effects. There are few epidemiological studies of the health effects of Texanol and TXIB, but one study found associations between the concentration of Texanol and TXIB in indoor air in schools and asthma symptoms among students. A number of glycol ethers are used in various building materials and interior materials such as paints, adhesives, and floor polish. There is an increased concern about exposure to glycol ethers because these compounds are suspected to contribute to allergic disease. Moreover, PGE compounds (propylene glycol and glycol ethers) are hormone-disrupting chemicals.
Total volatile organic compounds (TVOC) is a summary measure for many different volatile organic compounds. Despite a recommendation not to use TVOC as a risk indicator for health effects and discomfort problems in buildings, the TVOC concept is still used. Studies have reported positive associations between TVOC levels in the home environment and asthma, atopic dermatitis and atopic disease in infancy, wheeze, nasal allergy, eye symptoms, respiratory symptoms, and SBS symptoms.
Phthalates is a group of substances used as plasticizers to soften plastics, as well as in other building materials. Phthalates are not chemically bound to the PVC polymer and can therefore be easily emitted to the environment. These compounds are found almost everywhere in the environment. One study showed that about 0.5–1% of the floor dust in schools and preschools consisted of the phthalate DEHP and that the content was higher in schools and preschools as compared to homes and offices in Sweden. The majority of the epidemiological studies on indoor exposure to phthalates have studied asthma, allergies, and lung function. There are few longitudinal studies investigating the development of morbidity in relation to phthalate exposure indoors, but some studies exist linking levels of phthalate metabolites in urine to the development of asthma and allergy. There are few epidemiological studies on indoor exposure to phthalates in relation to health effects other than asthma and allergy.
Brominated flame retardants are a diverse class of semi-volatile organic compounds. PBDEs (poly-brominated diphenyl ethers) and HBCD (hexa-bromocyclododecane) have been measured in indoor air and indoor dust in many countries. Studies that have examined the health effects of PBDE exposure have focused on endocrine disruption and neurodevelopmental effects, but few of these studies have investigated the health effects of indoor exposure to these compounds. Due to suspected health effects of PBDEs, they are subject to regional as well as international restrictions. There is an increased use of organophosphorus compounds (OPs) as alternative flame retardants due to the restrictions on the use of PBDEs. Despite the widespread use of OPs in indoor environments, few studies exist on health effects in humans or animals. Indoor dust can be an important source of exposure to flame retardants, and small children might have a higher exposure than adults because they spend more time on the floor level and because of their hand to mouth behaviour.
The indoor levels of chemicals in the air may be higher in indoor environments with new materials. Due to this experience, "indoor painting", "renovation", etc., have been used in epidemiological studies as proxy variables for exposure to such chemicals. A number of studies have shown an association between interior painting at home and asthma or asthma symptoms, but also for other symptoms such as headache and fatigue. Most of these studies are cross-sectional, but there are some longitudinal studies. The more recent studies on this topic are often from Asia (China, Taiwan, and Japan). There are also a number of studies demonstrating associations between renovation of the residence and asthma or asthma symptoms, rhinitis (nasal symptoms), and dermatitis (dermal symptoms) in both children and adults. Although these studies often are conducted in Asia, there are also studies from Europe (Sweden, Germany, and France) showing similar associations as in Asia. Because the chemical content of paints and building materials varies between countries, relationships might also vary between countries. Moreover, some studies from Asia have reported that emissions from new furniture can be a health problem in terms of respiratory symptoms and atopic dermatitis. A probable cause is the emission of formaldehyde, but other VOC emissions can also contribute to these findings. No health studies were found on chemical emissions from new furniture in Europe or the US.
There are several studies on associations between the presence of PVC materials in the home, especially PVC flooring in the bedroom, and wheeze as well as doctor-diagnosed asthma in children. However, few studies exist on the associations between PVC material in the indoor environment and other health effects, such as hormonal effects. Studies on the health effects of PVC materials in the indoor environment are mostly based on self-reported data on the presence of PVC materials from questionnaires, but validation studies have shown that it is difficult to gather relevant information about PVC in the home environment through questionnaires. Despite this limitation, there are two studies that have shown that self-reported presence of PVC in the home is associated with increased levels of certain phthalates in dust or their metabolites in urine.
Studies from various countries have demonstrated an association between odour in the home environment, other than mould odour, and asthma, rhinitis, and SBS symptoms. Because all of these studies are cross-sectional studies, it is not possible to draw any conclusions on causality between odour in the home and respiratory illness or SBS symptoms. It is possible that some of these associations are due to worsening of pre-existing morbidity, such as asthma and rhinitis, by the odour.