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Contaminants of indoor air can be classified either according to the source (tobacco smoke, emissions of construction materials, microbial toxins) or their physical/chemical characteristics (e.g. particulate matter, hydrocarbons, phthalates). Indoor contaminants are an extremely variable group of compounds with the only common factor that the indoor air is their typical route of exposure. Thus, there will be huge variations in the relative importance of factors influencing exposure. The main groups are particulate, gaseous and semivolatile organic compounds from materials, and metabolic products made by microbes in moisture-damaged buildings. The latter can be either volatile compounds or chemical compounds present as tiny particles.


====Sources====

Indoor chemicals originate from both outdoor and indoor sources. Indoor sources include all kinds of structural and finishing materials, chemicals used in the home or workspace such as detergents, cleaning chemicals, solvents, clothes, utensils, and food ingredients like flour. Human beings emit themselves also a number of compounds. Space ventilation is intended to remove impurities from indoor space and prevent any noticeable exposure but in practice this does not usually happen.

Chemical impurities may be volatile organic compounds (VOC) or semivolatile organic compounds (SVOC) in the air, or they can be bound to particulate matter. Volatile compounds include aliphatic or aromatic halogenated hydrocarbons, alcohols, aldehydes, ketones, esters and ethers. Compounds evaporate to various degrees from almost all materials, consumer products and upholstery. Some volatile materials in the indoor air originate from outdoor sources (e.g. benzene from traffic). Even energy saving can cause problems, since lamps containing mercury may break.


====VOC, volatile organic compounds====

There are scores of various volatile compounds in the indoor air, but individual compounds are most often present at very minute concentrations. There are many factors which can influence the concentrations, and there have been no comprehensive studies published on the distribution of different VOCs and SVOCs in the indoor air. Emissions of construction materials provide a general picture of what kinds of compounds could be present indoors.

The health effects of indoor VOCs are poorly known. There are data on the health effects on individual compounds at high concentrations e.g. in workplaces, but concentrations in indoor environments other than workplaces manufacturing or using the chemicals are much smaller. One major gap in our knowledge concerns the long-term effects of simultaneous exposure to many compounds.


====SVOC, semivolatile organic compounds====

Semivolatile compounds in indoor air and dust include PAH-compounds (polyaromatic hydrocarbons), pesticides, flame retardants and plastic softeners. All combustion processes<ref>Indoor smoke, see the chapter "[[Can environmental cigarette smoke cause lung cancer?]]"</ref> and bitumen and tar containing paints are sources of PAH-compounds. PCB compounds can be found in fluorescent lamps, paints, sealants and fillings of structural elements, where they are used as flame retardants. Phthalates are common plastic softeners, they are also generally found in textiles, wallpapers, glues, paints and printing colours. The organic flame retardants include organophosphate compounds, chlorinated organophosphate compounds and certain brominated compounds.

Emissions of and exposures to SVOC compounds have been only recently studied, and therefore information is very scanty. However, SVOC emissions may behave differently from VOC emissions; e.g. flame retardant emissions from a TV-set may increase as the device ages, while most VOC emissions are greatest from brand new products. SVOC compounds are mostly in the particulate form, because they condensate soon after being emitted because of their high boiling points.

There is really insufficient data on the health effects of SVOC compounds. In a recent study, the phthalate concentrations in indoor dust were associated with respiratory symptoms in children. However, much more information is needed before we can assess the importance of these compounds as indoor pollutants.

The general advice to consumers is to take care of ventilation in homes (ventilation rate at least 0.5 meaning that one half of the volume of room air is exchanged within one hour). Newly built or recently repaired homes should be ventilated more effectively during the first months after the completion of the work.


'''''We know far too little about compounds in the indoor air, irrespective of whether they are synthetic compounds from construction materials or “natural” compounds from living organisms. Since people can choose what they eat but they cannot choose the air they breathe, the discrepancy between food control and indoor air and materials control is obvious.'''''



'''Notes and references'''
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