Several of the VOCs present in indoor air have caused cancer in animal studies when the animals were exposed to high concentrations. A few of these VOCs, for example formaldehyde and benzene, are considered by many authorities to be proven or probable human carcinogens [2, 29, 30]. In 1996 and again in 2006, the International Agency for Research on Cancer (IARC) of the World Health Organization concluded that formaldehyde was a human carcinogen . The primary evidence supporting this conclusion was increases in cancer in workers exposed to formaldehyde concentrations exceeding the concentrations present in most non-occupational indoor environments. The formaldehyde concentrations necessary to cause cancer were not specified but the IARC; however, the committee's conclusions were based primarily on studies of industrial exposures to formaldehyde with concentrations of 500 ppb or higher. The study cited by the IARC as " most informative" found that cancer risks were primarily associated with time-average workplace concentrations greater than 500 ppb and peak workplace concentrations greater than 2000 ppb .
The cancer risks from exposures to these VOCs have been estimated using exposure and risk assessment models. Some of these estimates rely on extrapolations from animals to people and all estimates rely on extrapolation from high (e.g., industrial occupational) to low pollutant doses. The estimation process generally uses conservative assumptions that tend to overestimate cancer risks. In the field of cancer risk assessment, the general precautionary assumption has been that cancer risks increase or decrease in direct proportion to the magnitude of pollutant exposure with no threshold concentration (concentration below which there is no increased cancer risk). If this assumption is correct, indoor exposures to known carcinogens must be presumed to pose some risk of cancer.
A recent analysis  estimated the cancer risks of a broad range of organic hazardous pollutants in the U.S. non-smoking population that is not occupationally exposed to these pollutants. Many of these hazardous pollutants are VOCs. For some of these VOCs such as formaldehyde, the sources are predominately indoors. For other VOCs, the sources are predominately outdoors, although the majority of exposure may still occur indoors due to the entry of polluted outdoor air into buildings where people spend most of their time. For all of the hazardous pollutants considered, the total cancer risk was approximately 1 in 1,000 based on cancer potency estimates from the U.S. EPA . The total estimated risk was 0.6 in 1,000 based on the cancer potency estimates of the California EPA . Sixty nine percent of the total estimated risk was from indoor exposures and 35% of the total risk was from pollutants emitted indoors. The VOCs predominately from indoor sources that posed the largest risks were formaldehyde (approximately one in 10,000 risk), and naphthalene and chloroform (both with a risk of approximately one in 100,000). The estimated risk from paradichlorobenzene (1,4-dichlorobenzene), substantially or largely from indoor sources , was also approximately one in 100,000. A similar risk assessment was performed for inner-city teenagers in New York City and Los Angeles . For the 13 VOCs selected, the mean total estimated risk from all sources of exposure (indoors and outdoors) was approximately 1 in 1,000 in both cities. The largest sources of risk were paradichlorobenzene, formaldehyde, chloroform, acetaldehyde, and benzene. For all of these VOCs except benzene, the primary sources of exposure were indoors. More than 40% of the total risk was attributed to sources in homes.
Considerable attention has been devoted to estimating the cancer risks from inhalation of formaldehyde and the estimated risk varies widely. EPA's Integrated Risk Information System  provides an estimated cancer risk of one in 10,000 at a concentration of 7 ppb. Risks are assumed to vary in direct proportion to concentration. Liteplo and Meek  reviewed the evidence of cancer risk of formaldehyde versus airborne concentration. Their review was based heavily on laboratory studies of how formaldehyde affected cell proliferation in animals. They concluded that the risks to the general population of respiratory tract cancers from formaldehyde are "exceedingly low" for concentrations less than 80 ppb. As discussed above, typical indoor formaldehyde concentrations are significantly less than 80 ppb. While the authors do not precisely quantify the term "exceeding low", they refer to a risk estimate of approximately three in one hundred million. They also note that the more typical risk modeling method results in a 1000-fold higher estimate of risk. Another recent paper used sophisticated models of exposures and of cancer induction mechanisms to predict the risks of respiratory tract cancer from exposures to formaldehyde . For non-smokers, the predicted risk of cancer with an 80-year exposure to a formaldehyde concentration of 30 ppb was one in ten million to 30 in one million depending on the model used to relate cancer dose with risk. For smokers, the corresponding risk estimates were approximately one in one million to 500 in one million.
In summary, some indoor VOCs are designated by multiple authorities as human carcinogens. Estimates of the magnitudes of cancer risks posed by these VOCs vary widely. The cancer risks posed by indoor-generated VOCs appear to be comparable in magnitude to the cancer risks of exposures to VOCs from outdoor air. Given the uncertainties in cancer risk assessment, particularly the uncertainties of extrapolating from occupational-level concentrations (e.g., 500 to 2000 ppb for formaldehyde) to typical indoor concentrations (e.g., a few 10s of ppb for formaldehyde), the magnitude of the cancer risks posed by indoor VOCs will continue to have a high level of uncertainty.
Given that indoor VOCs, as discussed above, may significantly increase the risks of cancer, it is useful to maintain an awareness of the indoor sources of the VOCs posing the greatest risk. Table 2 lists the VOCs indicated in the papers referenced above as the largest sources of cancer risk and their main indoor sources. Reducing or eliminating these sources, when feasible, is an option for those who wish to minimize cancer risks from indoor VOCs.
|VOC||Examples of Indoor Sources||Reference(s)|
|formaldehyde||some manufactured wood products used as building materials, in cabinets, and in furniture (e.g., medium density fiberboard, particle board, plywood with urea formaldehyde resin; urea formaldehyde foam insulation (no longer used but still present in some buildings); tobacco smoking; ozone-initiated chemical reactions with common indoor VOCs, unvented combustion appliances||[3, 4, 37, 38]|
|napthalene||pesticides (moth balls)|||
|paradichlorobenzene||pesticides (moth crystals); toilet bowl deodorizer||[1, 2, 32]|
|chloroform||pesticides; showering; washing clothes and dishes||
|acetaldehyde||tobacco smoking; water-based paint; unvented combustion appliances; leakage from wood stoves, furnaces, and fireplaces; (outdoor air also an important source)||[1, 3]|
|benzene||tobacco smoke; some furnishings, paints, coatings, wood products, gasoline from attached garages (outdoor air also an important and sometimes predominant source)|||