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ArticleTechnical Article

How Common is MCS (Multiple Chemical Sensitivity)?

From "A National Population Study of the Prevalence of Multiple Chemical Sensitivities," published in the Archives of Environmental Health, Volume 59: Number 6, June 2004.


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This study determined the prevalence of chemical hypersensitivity and the medical diagnosis of multiple chemical sensitivities (MCS) in the American population. The etiology and symptomatology of MCS were also investigated. A telephone survey of 1,054 randomly selected individuals within the continental United States was used, which produced a 95% confidence level and a +/– 3% confidence interval.


"The study found that 11.2% of Americans reported an unusual hypersensitivity to common chemical products such as perfume, fresh paint, pesticides, and other petrochemical based substances..."

The study found that 11.2% of Americans reported an unusual hypersensitivity to common chemical products such as perfume, fresh paint, pesticides, and other petrochemical based substances, and 2.5% reported being medically diagnosed with MCS. Additionally, 31.1% of the sample reported adverse reactions to fragranced products, and 17.6% experienced breathing difficulties and other health problems when exposed to air fresheners. While chemical hypersensitivity was more common in females, it affected individuals in all demographic groups studied.


Despite concerns that hypersensitivity to low levels of common chemicals is a ubiquitous public health problem, until now its prevalence in the American population had not been established. Chemical hypersensitivity, often called multiple chemical sensitivities (MCS), is also referred to as toxicant-induced loss of tolerance or environmental illness. It is typically acknowledged to be a condition characterized by acute reactions that occur after exposure to even low levels of common chemical products such as fragrances, household cleaners, fresh paints, newsprint, pesticides, and other products that contain petrochemicals. MCS can produce a wide range of symptoms, and individuals with the hypersensitivity can encounter great difficulty functioning in normal working and living environments.

While a limited number of epidemiological studies had investigated the regional prevalence of chemical hypersensitivity, its national prevalence had been speculative. The National Academy of Sciences estimated that up to15% of the American population could experience some degree of hypersensitivity to common chemicals. The California Department of Health Services found a prevalence in California of 15.9%, while similar studies in the Atlanta, Georgia metropolitan area and the state of New Mexico found a prevalence of 12.6% and 16% respectively. These three studies used accepted epidemiological methodology with randomly derived samples. Other published prevalence studies, which used methodologies such as anecdotal evidence, non-randomly or self-selected subjects, or a broader definition of chemical hypersensitivity, have produced varying findings. For instance, a study that relied exclusively on conversations with medical personnel in clinical settings estimated that 2% to 10% of the population experiences hypersensitivity. Two subsequent studies in Arizona that used self-selected subjects, young adult college students and elderly persons, determined that approximately 15% of the younger group and more than 37% of the elderly group reported a hypersensitivity to chemicals. A population survey in rural North Carolina, using a broader definition of MCS that did not distinguish between adverse reactions to harsh chemical odors and a true hypersensitivity, found a rate of 33%.

Research suggests that MCS exhibits a two-step process. The first step, initiation, is when the hypersensitivity first develops. Initiation can result from an acute exposure to a particular toxic agent, a chronic exposure to one or more toxic substances, even at low levels, or combinations of those mechanisms. The second step, triggering, is characterized by adverse physical reactions that occur after an exposure. Once the hypersensitivity has developed, symptoms can be triggered by a low-level exposure to a wide range of toxic substances – usually much wider than the initiating substance(s).

Other studies have hypothesized that MCS is linked to psychological factors and is an idiopathic disorder, which is either somatized or a conditioned response. These studies, however, have been criticized for methodological weaknesses, such as lack of pre-onset data, biased case selection.


This study uses a population survey to investigate the national prevalence of chemical hypersensitivity and its etiology, symptomatology, diagnoses, and lifestyle effects. To assure a valid national sample, cases were obtained by using random digitally dialed phone numbers, derived from the continental United States, and regionally weighted for representativeness. In order to achieve a 95% confidence level and a +/- 3% confidence interval (generally accepted levels for a random population study), the sample would contain 1,067 cases, based on the standard formula for determining population characteristics based on a proportion: n = {proportion x (1–proportion) x Z2} / {(confidence interval) 2} where n is the sample size, the proportion is 0.5, and Z is the statistical Z-score that corresponds to the confidence level. This study ultimately yielded 1,054 valid cases, which was within accepted bounds (0.02%) for the desired confidence interval (+/- 3%) at a 95% confidence level.



A 22-item questionnaire was used as the survey instrument. A longer questionnaire was initially developed, but a pretest found that it caused excessive respondent fatigue, forcing several respondents to terminate prematurely their participation. The original questionnaire therefore was shortened to maximize participation. A pretest of the 22-item questionnaire revealed no significant problems, and the measurement instrument was judged acceptable. External validity was promoted by constructing the questionnaire to conform to recommendations made by the federal Interagency Workgroup on Multiple Chemical Sensitivity, thus allowing comparisons with subsequent studies. The questions replicated ones in the regional studies, and followed recommendations from other studies to facilitate validity evaluations.


To insure reliability and to control for potential seasonal variations, the questionnaire was administered to four different seasonal cohorts. Data for the four cohorts were obtained in the spring of 2002, summer of 2002, fall of 2002, and winter/spring of 2003, respectively. The number of cases in each seasonal cohort varied somewhat, but the rates of a positive response to the key hypersensitivity question in each cohort (10.16%, 10.36%, 11.4%, 16%) were all within the desired confidence interval. After the data from each cohort were assessed and deemed statistically congruent with the others, the data from all four cohorts were aggregated to produce cumulative averages.



The reliability of the questionnaire was also evaluated by utilizing statistical measures of internal consistency. Measurements of Cronbach’s coefficient of alpha and other measures of inter-item correlation were used on a cluster of related questions to determine consistency levels of the responses. A subsequent item analysis was conducted to further evaluate the findings and promote the integrity of the study. For the Cronbach and other inter-item analysis, questions about behavior modifications were clustered. An additional cluster consisting of questions about behavior modifications and changing foods was also used.


The initial question asked all respondents if they had ever been medically diagnosed with asthma, and 14.1% of the sample responded “yes.”  A subsequent question asked all respondents if they suffered from allergies to natural substances, such as dust, mold, grass, pollen, and animal dander, and 36.4% of the sample responded “yes.” The next question asked all respondents about “air fresheners” and found that 17.6% reported getting headaches, breathing difficulties, or other health problems from them. All respondents were also asked if being next to someone who was wearing a scented product was irritating or appealing, and 31.1% said “irritating,” 46.3% said “appealing,” and 21.7% said “not sure.”

The key question on chemical hypersensitivity replicated the wording used in both the California and Atlanta studies, and asked respondents the following: “Compared with other people, do you consider yourself to be allergic or unusually sensitive to everyday chemicals like those in household cleaning products, paints, perfumes, detergents, insect sprays, and things like that?” Among all respondents, 11.2% answered “yes” to this key question with an additional 2.6% saying “not sure." The next question asked all respondents if they had ever been diagnosed by a medical professional as having MCS, and 2.5% answered “yes.”

Chemically hypersensitive individuals have reported a variety of symptoms and reaction dynamics in other studies. Thus, the respondents in this study who answered “yes” to the key hypersensitivity question were asked follow-up questions to assess the characteristics and magnitude of their symptoms. The first follow-up question asked about the severity of the hypersensitivity. The percentage of hypersensitive respondents that characterized their symptoms as severe was 23.6%, with 43.1% saying moderately severe, 26% saying mild, and 6.5% reporting that they didn’t know. The hypersensitive respondents were also asked if they had developed sensitivities to certain foods since the emergence of their sensitivity to chemicals, and 18.5% said “yes.”

Individuals with the hypersensitivity were then asked about lifestyle changes after the onset of their hypersensitivity.


"Respondents were asked if their hypersensitivity made it difficult to shop in stores or be in public places such as restaurants, houses of worship, or theaters. This question received a 39.5% positive response."

Respondents were asked if their hypersensitivity made it difficult to shop in stores or be in public places such as restaurants, houses of worship, or theaters. This question received a 39.5% positive response . A subsequent question found that 72.7% of the hypersensitive respondents had changed their household and personal care products. Also, 13.2% reported losing their job, and 6.7% had moved from their home because of their hypersensitivity. 


When respondents with hypersensitivity were asked how long they have had their symptoms, 5.2% said less than one year, 25% reported one to five years, 20.7% were in the five to ten years range, 29.3% said ten to twenty years, and 19.8% responded more than twenty years. When asked how old they were when the hypersensitivity originally developed, 28.2% said under the age of 20, 28.2% said age 20-35, 21.4% said age 36-50, and 14.5% said over the age of 50, with 7.7% being unable to recall or didn’t know.



Etiology of the chemical hypersensitivity was also investigated. Respondents were asked if they either knew or had a good idea what caused their hypersensitivity, and 29.1% reported knowing the cause, 9.4% suspected the cause but could not be certain, and 58.1% could not identify the cause . Of those who could identify a potential cause, 18.5% reported that an exposure to solvents or cleaners initiated their hypersensitivity, while 4.6% reported an exposure to pesticides, 10.8% reported an exposure to building materials or furnishings, and 49.2% reporting some other cause.


Respondents with the hypersensitivity were also asked if they had ever experienced any serious emotional problems prior to the emergence of their hypersensitivity, and if any emotional problems had developed since its onset. Of the hypersensitive respondents, 10.3% reported having emotional problems before the hypersensitivity emerged, and 19.5% reported these types of problems only after the hypersensitivity developed. 



Demographic questions were asked of all individuals in the sample, and a cross-tabulation with the hypersensitivity was subsequently conducted. The age composition of the entire sample was as follows: 7.9% were under 20 years old, 25.8% were in the 20 to 35 range, 27.4% reported being in the 36 to 50 category, 36.8% said they were over 50, and 2.1% didn’t know or declined to answer. Regarding gender, males constituted 38.8% and females 61.2% of the sample. The educational levels in the sample were 10.3% reported having less than high school education, 28.6% were high school graduates, 26.2% had attended some college, 32.8% were college graduates and higher, with 2% answering not sure. The ethnicity/race composition of the sample was 2.1% Asian or Pacific Islander, 5.7% Hispanic or of Latin origin, 11.2% Black or African-American, 76.3% Caucasian or European, and 4.8% other or declining to answer.



The cross-tabulation of individuals who gave a positive response to the hypersensitivity question with the demographics of the entire sample produced the demographic characteristics of the hypersensitive. The gender of the hypersensitive respondents was 13.8% male, and 86.2% female. The ages of the hypersensitive respondents were 5.5% under 20 years, 18.2% in the 20 to 35 years grouping, 25.5% in the 36 to 50 years range, and 50.9 over 50 years old. The educational levels of the hypersensitive respondents were 10.5% less than high school, 27.2% high school graduates, 26.3% some college, 33.3% college graduates and higher, and 2.6% declined to answer. The ethnicity/race breakdown of the hypersensitive respondents were 0% Asian/Pacific Islander, 5.3% Hispanic/Latin Origin, 13.2%, Black/African American, 78.9% Caucasian, and 2.6% other or declining to answer.


The 11.2% national prevalence of chemical hypersensitivity found in this study is statistically congruent, given the confidence level of +/- 3%, with the regional prevalence of 12.6% and 15.9% found in the Atlanta and California studies, which used the same key question on hypersensitivity. The 31.1% of this sample that found scented products on other people to be irritating also supports the prevalence rate of 33% found in the North Carolina study that used a broader definition of chemical hypersensitivity.

While the cross-tabulation of demographic variables and hypersensitivity indicate that a disproportionate number of females and older individuals report chemical hypersensitivity, these findings are less significant when the gender (38.9% male, and 61.1% females) and age (36.8% over 50) bias in the entire sample is considered. Additionally, while older people are more likely to report hypersensitivity, only14.5% of the hypersensitive respondents said that their symptoms first appeared after age 50, while 56.4% reported that their symptoms first developed under age 35. This finding indicates that chemical hypersensitivity affects a wide age range and is not limited to older individuals. A comparison of the demographic characteristics of the entire sample with those of the hypersensitive shows close conformance among the percentages in both groups, indicating that chemical hypersensitivity is widely distributed throughout the general American population cutting across racial/ethnic, age, and gender groupings.

An internal consistency analysis, which used a clustering the behavior variables of difficulty shopping, changing personal care products, loss of employment, and the necessity to change housing, produced a Cronbach alpha coefficient of 0.6189. When the same cluster was used with the additional variable of changing food, the alpha coefficient rose to 0.6622. These results suggest that respondents with chemical hypersensitivity have taken varying actions to cope with their symptoms. While 72.7% have changed their personal care products, for example, only 6.7% have found it feasible to move from their home.  

The results from the question on pre-hypersensitivity emotional problems have etiological implications. That only 10.3% of the hypersensitive respondents had a history of emotional problems prior to the onset of their symptoms, and that hypersensitivity cuts across all demographic categories, weakens the notion that MCS is somatized. While some respondents may have been reluctant to admit previous emotional problems, 19.5% reported experiencing problems after the symptoms appeared, suggesting a reasonable level of respondent veracity. This study’s findings, therefore, suggest that chemical hypersensitivity is inconsistent with psychogenic disorders. 



The findings of this study provide additional evidence that hypersensitivity to low levels of common chemicals is a widespread phenomenon in the American population. It is widely distributed across racial/ethnic, age, and educational groups. Of the 11.2% of respondents who experience chemical hypersensitivity, 66.7% describe their symptoms as either severe or moderately severe, indicating that chemical hypersensitivity poses a significant public health problem. Consequently, the dynamics, etiology, and symptomatology of MCS deserve substantially more research and attention.



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How Common is MCS (Multiple Chemical Sensitivity)?:  Created on April 24th, 2007.  Last Modified on January 11th, 2010


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Other Articles by Stanley M. Caress, Ph.D. and Anne C. Steinemann, Ph.D.

About Stanley M. Caress, Ph.D. and Anne C. Steinemann, Ph.D.

Stanley M. Caress, Ph.D. is a Professor with the State University of West Georgia.

Anne C. Steinemann is Professor of Civil and Environmental Engineering, Professor of Public Affairs, and Director of The Water Center at the University of Washington.

Together, they authored the article, "A National Population Study of the Prevalence of Multiple Chemical Sensitivities," published in the Archives of Environmental Health, Volume 59: Number 6, June 2004.




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