The national prevalence of chemical hypersensitivity, the medical diagnosis of multiple chemical sensitivities , and potential overlaps with asthma

Townsend Letter for Doctors and Patients, August-Sept, 2005 by Stanley M. Caress, Anne C. Steinemann

Abstract

A national population study, using a random sample of 1,057 geographically weighted cases, was conducted to determine the prevalence of chemical hypersensitivity, the medical diagnosis of multiple chemical sensitivities (MCS), (1) and their co-occurrence with asthma in the American population.

The study found that 11.2% of the sample reported a hypersensitivity to chemicals, and 2.5% reported being diagnosed with MCS. Additionally, 14.1% of the respondents reported being diagnosed with asthma. Of those with asthma, 27% reported also being hypersensitive to chemicals and 7.4% reported also being diagnosed with MCS. Of those diagnosed with MCS, 42% reported also being diagnosed with asthma.

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Additionally, 31.1% of all respondents reported an aversion to scented products, and 17.6% of all respondents experienced negative physical reactions to air fresheners. Among those with asthma, 37.2% found scented products irritating and 29.7% said air fresheners caused breathing difficulties, both of which are common symptoms of MCS.

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Chemical hypersensitivity, often medically diagnosed as multiple chemical sensitivities (MCS), (1) is also known as toxicant-induced loss of tolerance (2) and environmental illness. MCS is characterized by adverse physical reactions to chemicals in common products such as household cleaners, laundry supplies, fresh paint, fragrances, synthetic building materials, new carpets, pesticides, and other petro-chemically based products. MCS is considered permanent and the only way that sufferers can prevent reactions is to completely avoid problematic chemicals. MCS reactions, which range from mild to severe, can result from exposures to even low levels of the irritating substances. (3)

Research indicates that MCS exhibits a two-step process. The first step, initiation, is when the hypersensitivity first develops. Initiation can result from a single high-level exposure to a particular toxic agent, or it can emerge after chronic exposure to one or more toxic substances, even at low levels. The second step in MCS, triggering, is when adverse physical reactions occur as a result of an exposure. Once the hypersensitivity has developed, symptoms can be triggered by an exposure to a range of offensive substances, usually much wider than the initiating substance, and even at very low levels.

In addition to growing concern about MCS, there has been mounting alarm about the increasing occurrence of asthma. This has stimulated researchers to investigate asthma's linkage with other conditions. (4) Recent studies have explored the possible association of certain types of asthma with chemical hypersensitivity. (5) These studies have suggested that chemical hypersensitivity can be a symptom of some forms of asthma. (6) Asthma has numerous phenotypes based on age of onset, etiology, and characteristics. (7) Occupational asthma, one of the phenotypes, originates from workplace exposures to toxic substances. Occupational asthma is further categorized based on how rapidly the condition emerges. Occupational asthma can be either "with latency" or "without latency." Occupational asthma with latency develops over time when continued exposure to a toxic substance creates allergic sensitization, while occupational asthma without latency occurs immediately after a single massive toxic inhalation. (8) Reactive airways dysfunction syndrome (RADS), which has also been termed irritant-induced asthma, (9) is a subset of occupational asthma without latency. (10) RADS is a persistent, airways hyperresponsiveness that occurs after a single high-level toxic exposure incident. RADS is a permanent condition (14) and once sensitized, individuals with it negatively react to much lower levels of the irritating substance.

RADS sufferers frequently exhibit chemical hypersensitivity, which has led some researchers to conclude that RADS and MCS are overlapping disorders. (15) Apparent similarities between MCS and RADS provided the impetus for a study utilizing animal models, which found that asthma and MCS have common etiologies and characteristics. (16) An additional study, which focused primarily on symptomatology, indicated that specific forms of asthma and MCS have similar dynamics which frequently overlap. (17) Thus, it is becoming evident that MCS has commonalities with at least some types of asthma.

While there is increasing evidence that chemical hypersensitivity is a symptom of some categories of asthma, there have been no prior national population studies of the co-occurrence of asthma and chemical hypersensitivity or of asthma and the medical diagnosis of MCS. There have been prevalence studies, however, of chemical hypersensitivity and the medical diagnosis of MCS in the American population, and separate epidemiological research on asthma.

MCS Prevalence

This study is the first to investigate the national prevalence of chemical hypersensitivity and the medical diagnosis of MCS using a randomly selected population-based sample. Prior to this study, the National Academy of Sciences estimated that up to 15% of the American population could experience some degree of hypersensitivity to common chemicals. (15) Regional studies have also been conducted. The California Department of Health Services found a MCS prevalence in California of 15.9% (n=4046), (16) while similar studies in the Atlanta, Georgia metropolitan area and the state of New Mexico found rates of 12.6% (n=1579) (17) and 16% (n=1813) respectively. (18) These three studies used accepted epidemiological methodology with randomly derived samples. Other published studies of the prevalence of chemical hypersensitivity have used anecdotal evidence, non-randomly or self-selected subjects, or a much broader definition of hypersensitivity, and they produced a wide range of findings. A study that relied exclusively on conversations with medical personnel in clinical settings estimated that between 2% and 10% of the population experiences chemical hypersensitivity. (19) Two subsequent studies in Arizona used self-selected subjects, young adult college students and elderly persons, and determined that approximately 15% (n=809) of the younger group (20) and more than 37% (n=160) of the elderly group (21) reported hypersensitivity to chemicals. A population survey in rural North Carolina, using a broader definition of chemical hypersensitivity that did not distinguish between an aversion to harsh chemical odors and true hyperreactiveness, found a rate of 33% (n=1027). (22)

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The national prevalence of chemical hypersensitivity, the medical diagnosis of multiple chemical sensitivities , and potential overlaps with asthma

Townsend Letter for Doctors and Patients, August-Sept, 2005 by Stanley M. Caress, Anne C. Steinemann

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