Introduction to VOCs

In the field of IAQ research, the term "volatile organic compound" or "VOC" refers to any of thousands of organic (carbon-containing) chemicals that are present mostly as gases at room temperature. Inorganic carbon-containing gases such as carbon dioxide and carbon monoxide are excluded from this definition. VOCs can be man-made or naturally occurring chemical compounds. VOCs include a very wide variety of types of molecules that can be categorized in many ways, such as by structure (e.g., straight-chained, branched, ring structures), by the types of chemical bonds (alkanes, alkenes, alkynes, saturated, unsaturated), by the function of specific parts of the molecules (e.g., aldehydes, ketones, alcohols, etc.), or by specific elements included (e.g., chlorinated hydrocarbons that contain chlorine, hydrogen, and carbon).

An important subgroup of VOCs is semi-volatile organic compounds or SVOCs which tend to have a higher molecular weight and higher boiling point temperature than other VOCs. Examples include plasticizers, flame retardants, and pesticides. All indoor VOCs are present partly as gaseous airborne chemicals and partly as chemicals adsorbed on indoor surfaces and onto microscopic airborne and settled particles. SVOCs are often present largely on surfaces and particles, with only a small fraction in the air unattached to particles.

The amount, or concentration, of VOC present in the indoor air is expressed in a variety of units. Commonly used units are parts per billion (ppb), parts per million (ppm), and micrograms per cubic meter (µg/m³). A microgram is one one-millionth of a gram. If the concentration is 1 ppb (or 1 ppm), for every billion (or million) molecules of air there is one molecule of the VOC. If the concentration is 1 µg/m³, then for every cubic meter volume of air there is 1 microgram of mass (weight) of the VOC.

A large number of VOCs are emitted into indoor air from building materials, furnishings, cleaning compounds, office equipment, personal care products, air fresheners, pesticides, occupant activities, and unvented combustion processes such as tobacco smoking, burning of wood or kerosene, or cooking with gas stoves [1-5]. Some of the key indoor sources of SVOCs are pesticides, building or decorating materials made of or containing flexible plastics such as vinyl wallpaper or vinyl flooring, and building materials and furniture containing flame retardants.

VOCs are also produced indoors from chemical reactions of indoor ozone with other VOCs, SVOCs, or materials (such as carpeting) [6]. Outdoor air is normally the major source of the indoor ozone, although ozone, ion, and plasma generators (marketed as air cleaners), electronic air cleaners (that unintentionally produce ozone as a by-product), and some types of office equipment can be additional sources of ozone. VOCs also enter buildings along with outdoor air; however, for many VOCs and SVOCs for which the primary sources are indoors, indoor air concentrations far exceed outdoor air concentrations [1, 2]. Most VOCs indoors are at low concentrations, but the concentrations due to indoor sources are highly variable and depend on occupant behaviors. Some indoor VOCs can be at high concentrations. For example in some homes ethanol concentrations are above 1000 µg/m³, and in some homes 1,4-dichlorobenzene, alpha-pinene, and d-limonene concentrations have been measured at around 100 µg/m³ [7]. Information is available from representative U.S. population surveys on the distribution of personal exposures to a variety of VOCs [8].

Researchers, and those who investigate indoor air quality problems, sometimes measure and report "total volatile organic compound" or "TVOC" concentrations. The term TVOC refers to the total concentration of multiple airborne VOCs present simultaneously in the air. TVOC methods do not measure all VOCs in the air, but a subset of VOCs that are expected to be present. Measuring TVOC concentrations is less expensive than measuring the concentrations of many individual VOCs. However, there are two main limitations to TVOC measurements. First, different TVOC measurement methods can yield substantially different TVOC concentrations and the differences between measurement methods will depend on the mixture of VOCs present. Secondly, the toxicity and the odor thresholds of individual VOCs within the VOC mixture may differ by orders of magnitude; therefore, the total concentration is not likely to provide a useful measure of total toxicity or total odor level. In general, TVOC measurements in buildings have not been useful in predicting health effects,

Some VOCs and SVOCs are odorous and some are suspected causes of adverse health effects. The suspected health effects cover a broad range including, but not limited to, sensory irritation symptoms, allergies and asthma, neurological and liver toxicity, and cancer. While multiple VOCs present together may have effects greater (or less) than the sum of their individual effects, little information is now available on such combined effects. The following text briefly summarizes the current knowledge about the linkages of indoor VOCs with sensory irritation, allergies, asthma, and related respiratory effects, and cancer. Additional sections briefly summarize knowledge about potential health effects of VOCs in cleaning products, SVOCs, and VOCs produced indoors from chemical reactions.

One common VOC, formaldehyde, is widely used in the manufacture of building materials and numerous household products, and is also a by-product of combustion and other natural processes. Formaldehyde may be present in substantial concentrations both indoors and outdoors. Due to its ubiquitous nature and significant health effects, this website’s section on “Indoor Volatile Organic Compounds and Health” often provides discussions focused specifically on formaldehyde.

Various organizations have established guidelines or recommendations (none are legally enforceable limits) for maximum formaldehyde concentrations, based on examinations of the scientific literature. Table 1 provides examples of these guidelines, which are discussed again later in this document. It is evident in this table that, despite differences in guidelines from different organizations, the longer exposure periods (longer than 8 hours) consistently specify lower guideline concentrations of formaldehyde (7-40 ppb) relative to the guidelines for periods of 8 hours or less (44-750 ppb). An exception is the relatively high chronic guideline of 100 ppb from the World Health Organization (WHO). An alternate view of the evidence on chronic formaldehyde exposure and sensory irritation, apparently underlying the WHO guideline, is provided by Wolkoff and Nielsen [9]. These authors consider an indoor air quality guideline of 80 ppb, based on the available evidence, to be “protective against both acute and chronic sensory irritation in the airways in the general population . . . [9].” Nielsen et al., in 2013 [10], who had provided the basis for the 2010 WHO formaldehyde guidelines, reviewed recent evidence that they concluded strengthened support for the WHO guideline of 100 ppb as protective against all short- and long-term health effects, including sensory irritation, cancer, and reproductive effects.

Table 1. Guidelines and standards for formaldehyde.

Source	Concentration	Associated Period of Exposure	Health Effect(s)	Reference(s)
Based on sensory irritation
California Environmental Protection Agency (EPA)	44 ppb	1 hour	Eye and airway irritation	[11]^*
Health Canada	100 ppb	1 hour	Eye irritation	[12]
National Institute for Occupational Safety and Health	100 ppb**	15 minute	**	[13]
Occupational Safety and Health Administration	750 ppb	8-hour PEL-TWA	Cancer and skin/eye/ respiratory irritation	[14]
World Health Organization	81 ppb	30 minute	Sensory irritation	[15]
World Health Organization	100 ppb	Short- and long-term	Sensory irritation	[16]
Based on respiratory and asthma-like symptoms
Agency for Toxic Substances and Disease Registry	40 ppb 30 ppb 8 ppb	Daily:1-14 days 15-364 days > 1 year	Respiratory	[17] [18]
California EPA	7 ppb 7 ppb	8-hour annual average	Respiratory symptoms Respiratory symptoms	[11]^*
Health Canada	40 ppb (target)	8 hour	Respiratory symptoms in children	[12]
Based on cancer risk
National Institute for Occupational Safety and Health	16 ppb	8 hour	Nasal cancer	[13]
Occupational Safety and Health Administration	750 ppb	8-hour PEL-TWA	Cancer and skin/eye/ respiratory irritation	[14]
World Health Organization	100 ppb	Long-term	Nasal cancer	[16]