VOCs and Allergy, Asthma, and Related Respiratory Symptoms
The associations of indoor VOCs, SVOCs, and their sources with allergy and asthma outcomes and lower respiratory symptoms indicative of asthma have been investigated in many studies. Mendell  performed one of the earliest reviews of the scientific literature on this topic, focusing on exposures in homes and effects in infants and children. This review determined that some, but not all, studies found moderate to strong increases in respiratory or allergic health effects among children in homes with higher concentrations of selected VOCs or SVOCs, including formaldehyde and some types of phthalates (softening agents for some plastics), or in homes with larger amounts of some VOC or SVOC sources, such as particleboard, plastics, and recent paint. Because the reviewed studies could only demonstrate associations or correlations, but could not prove that the VOCs were causing the health effects, and because the number of available studies on each type of VOC was limited, the findings were presented at the time as only suggestive, but as justifying increased research.
A review by the California Environmental Protection Agency (EPA)  that considered a broad set of evidence (e.g., studies in homes, studies of workers, chamber exposure studies, animal studies, and research on the toxicological mechanisms) identified a linkage of formaldehyde with increases in asthma-like respiratory symptoms, which is the basis for their proposed 7 ppb guideline in Table 3.
|Source||Concentration||Associated Period of Exposure||Health Effect(s)||Reference(s)|
|Agency for Toxic Substances and Disease Registry||
> 1 year
|Health Canada||40 ppb (target)||8 hour||Respiratory symptoms in children|||
* REL developed using revised methodology .
McGwin et al.  found, in a systematic review and quantitative meta-analysis of studies on formaldehyde and childhood asthma, a significantly increased risk with two different analytical models providing odds ratios of 1.03 and 1.17 for a 10 µg/m3 increase of formaldehyde. McGwin identified weaknesses in the available literature and called for stronger and more homogeneous future studies to allow firmer conclusions. Le Cann et al. , in a review of the literature since 2006 on indoor environmental exposures and children's health, concluded that available studies supported a correlation between indoor VOCs and respiratory symptoms and allergic effects, and also a correlation between aromatic organic compounds and asthma.
In contrast, a review by Nielsen et al. , relying more on studies using animals and short-term chamber studies with humans, found little evidence that indoor VOCs (excluding formaldehyde) promoted development of airway allergies or asthma. A review by Wolkoff and Nielsen  did not find reported epidemiological associations between existing home formaldehyde concentrations and exacerbation of asthma in children and adults to be persuasive, because of possible alternative explanations due to other correlated but unmeasured indoor factors. A more recent literature review by Hulin et al.  found that evidence was too scarce to draw conclusions about relationships of formaldehyde and other VOCs, including SVOCs such as phthalates and flame retardants, with respiratory health.
A recent review of the literature on indoor exposures and asthma exacerbations  summarized current evidence related to formaldehyde, and to other VOCs. The review found only limited or suggestive evidence of an association between formaldehyde exposure and exacerbations of asthma, in particular through enhanced response to other allergens. This referred to two studies. In a blinded crossover study by  exposure to up to 92 µg/m3 formaldehyde for 30 minutes had no effect on lung function or symptoms in asthmatic adults, but enhanced both immediate and late responses to dust mite antigen. Formaldehyde thus increased effects of a common asthma trigger without having apparent direct effects. A similar lack of lung function effect of 500 µg/m3 formaldehyde exposure for 60 min was demonstrated in patients with intermittent asthma . For VOCs other than formaldehyde, the review  found that, although some studies have suggested associations between some indoor VOCs and respiratory effects, there was inadequate or insufficient evidence to determine whether or not an association exists between indoor residential VOC exposures and exacerbation of asthma.
Recent studies not included in the available reviews have fairly consistently identified health effects associated with indoor VOC exposures. Annesi-Maesano  reported that higher formaldehyde levels in French classrooms were associated with increased rhinoconjunctivitis (sneezing/runny nose and itching eyes), and there was in increased prevalence of past year asthma associated with high levels of acrolein. A French study reported, in two publications, that higher formaldehyde levels in homes were associated with increased risk of lower respiratory infections and increased night cough in infants [50, 51]. Choi et al.  reported that the total concentration of propylene glycol and glycol ethers (chemicals widely used in water-based paints, varnishes and cleaning fluids) in bedroom air was associated with increased risk of asthma, allergy, rhinitis, and eczema. In a subsequent paper by Choi et al. , the analyses revealed that the use of water-based cleaners and re-painting were associated with higher airborne concentrations of this class of VOCs. Raaschou-Nelson , however, found no association between long-term (18 month) exposure to formaldehyde in infant bedrooms and wheezing symptoms.
Elliot et al.  reported findings from a different kind of study, using a higher quality of measurements of both exposures and health outcomes, and in a representative U.S. population sample. Elliot et al. found that the highest exposures to 1,4-dichlorobenzene (1,4-DCB), as assessed by blood levels, were associated with substantial and statistically significant reductions in pulmonary function. Levels of 10 other VOCs did not show associations. The exposures influencing the blood levels of the participants might have been from home, school, work, or transit. The authors concluded that common exposures to 1,4-DCB (which come essentially entirely from indoor use of air fresheners, toilet bowl deodorants, and mothballs) may have long-term adverse effects on respiratory health. Using biomarkers in blood in this way can provide very accurate measurement of specific personal exposures; unfortunately, we do not yet have biomarkers for assessing most VOCs.
In summary, while additional research is needed before firm conclusions can be drawn about the effects of indoor VOCs or SVOCs on allergy, asthma, and related respiratory symptoms, the evidence of potential risks has been increasing and is now strongly suggestive of effects. The current evidence is also sufficient to indicate that more research on this topic is a high priority.