Environmental sensitives
The term Environmental sensitivities describes a variety of reactions to chemicals, electromagnetic radiation and other environmental factors at exposure levels commonly tolerated by many people. These phenomena are not yet fully understood. In contrast, some toxic environmental agents such as such as metals (e.g. lead, mercury), rock dusts (e.g. asbestos, silica), chemicals (e.g. hydrogen sulphide, dioxin) and biological agents (e.g. scorpion or snake venom) are better understood as to their ill effects on people.
"Environmental sensitivities" does not describe a single, simple condition with a universal cause. Environmentally sensitive individuals link their symptoms to aspects of their environment such as being in a particular place or being exposed to one or more factors such as chemicals, biological materials or electromagnetic phenomena.
Environmental exposures may not contribute to all these conditions in all patients, but one should be alert to the possibility that a range of factors may contribute to an individuals ill health.

# Aspects of Environmental Sensitivities
List of some terms that have been used to describe aspects of environmental sensitivities:
- State of heightened reactivity to the environment
- Total allergy syndrome
- Toxicant-Induced Loss of Tolerance (TILT)
- Multiple chemical sensitivity(ies) (MCS)
- Multiple chemical hypersensitivity(ies)
- Chemical intolerance(s)
- Gulf War syndrome/illness
- Idiopathic environmental intolerance
- Environmental illness
- Chemical injury/allergy
- Toxic injury
- Tight building syndrome
- Sick building syndrome
- Twentieth century disease
- Chemically induced illness
- Chemophobia
- Electromagnetic (hyper)sensitivities/intolerance
- Radiowave sickness

# Commonly Overlapping Conditions
Adding to the complexity of the clinical picture are overlapping conditions, for example:
- Myalgic encephalomyelitis (ME)
- Chronic fatigue syndrome
- Post-viral fatigue syndrome
- Post-infectious neuromyasthenia
- Yuppie flu
- Chronic pain
- Food intolerance syndrome
- Celiac disease
- Irritable bowel syndrome
- Major depression
- Anxiety or Panic disorder
- Allergies, Arthritis, Asthma, Fibromyalgia, Hypothyroidism, Migraine, Rhinitis
Given the complexities of the condition, the following section examines criteria for determining whether someone is experiencing environmental sensitivities.

# Diagnostic criteria
Diagnostic criteria are such that independent physicians would come to the same conclusion when examining a particular patient. This is important both for treatment purposes and for research.
With regard to multiple chemical sensitivity, thirty-four experienced North American physicians and researchers who had examined patterns of symptoms in thousands of people reached a consensus regarding criteria to establish a diagnosis:
- symptoms are reproducible with repeated exposure;
- the condition is chronic;
- low levels of exposure  result in manifestations of the syndrome;
- symptoms improve or resolve when the incitants(1)  are removed;
- responses occur to multiple chemically unrelated substances; and
- symptoms involve multiple organ systems.
A systematic literature review confirmed the diagnostic criteria, and suggested that neurological symptoms could be an additional criterion.The consensus diagnostic criteria were also validated, as they identified those most and least likely to be affected among 2,546 patients in Toronto medical practices with high and low prevalence of patients with sensitivities. In the same study, a combination of four neurological symptoms also discerned people most likely affected by multiple chemical sensitivities: having a stronger sense of smell than others; feeling dull/groggy; feeling "spacey;" plus having difficulty concentrating. A pattern consistent with these diagnostic criteria is also reported for sensitivities to electromagnetic phenomena.

# Prevalence
Diagnostic criteria are used by physicians to identify a health condition in individuals, and by researchers to determine the proportion of the population experiencing the condition severely enough to seek medical care. Some people with environmental sensitivities are less severely affected and may not seek care. This proportion of individuals in the population is generally investigated with more general questions about reactions to perfumes or other everyday chemicals.

## Canada
In January 2007, Statistics Canada reported that 5% of Canadians (1.2 million people) suffer "medically unexplained physical symptoms," including multiple chemical sensitivity, fibromyalgia and chronic pain.
According to Statistics Canadas 2003 National Population Health Survey (N=135,573)(2), the prevalence of doctor-diagnosed multiple chemical sensitivities was 2.4% in people aged twelve or older, and 2.9% in people thirty years of age or older. The 2005 National Survey of the Work and Health of Nurses revealed that 3.6% of all Canadian nurses experienced chemical sensitivities (N=18,676).

## USA
Prevalence in the general American population has been reported to range from 3.1% doctor-diagnosed multiple chemical sensitivity in Atlanta, Georgia (N=1,582)12 to 6.3% doctor-diagnosed chemical sensitivity in a large California survey (N=4,046).
Many more people experience less severe sensitivities. Self-reports of heightened sensitivity (feeling ill) on exposure to "everyday " chemicals in American populations ranges from 11% of 1,057 participants in a US national survey and 16% in California,13 to 33% in rural North Carolina. It is unclear to what extent people experiencing less severe intolerances are at an increased risk of developing full-blown, debilitating environmental sensitivities. However, recent studies have revealed genetic links to sensitivities, and biochemical differences between people with sensitivities and "control" populations. Clinical experience shows that increasing chemical exposures are associated with increasing symptoms and reports of sensitivity spreading to more incitants.

## Gender
More women than men are affected by environmental sensitivities. Almost twice as many women as men experienced "sick building syndrome" in a German study, and approximately 60-80% of people diagnosed with environmental sensitivities in various surveys are female.

## Socio-economic class
Environmental sensitivities affect all socio-economic classes, according to population-based surveys. The recent Canadian analysis indicates that people from a lower socio-economic class are more likely to report medically unexplained symptoms than are people from the highest socio-economic class.On the other hand, clinic-based and citizens groups surveys indicate that more highly educated or affluent people with sensitivities tend to seek medical care or self-help.

## Age
Sensitivities may occur in anyone, even at an early age. Childrens respiratory, learning and behavioural difficulties may be associated with toxins passed on from the mother, as well as a variety of factors including pesticide exposure, indoor air quality and foods.Studies have revealed that the prevalence of environmental sensitivities increases with age. For example, the prevalence of medically unexplained physical symptoms (chronic fatigue, fibromyalgia and multiple chemical sensitivity) in Canadians increases with age from 1.6% of people aged 12 to 24 years, to 6.9% in people 45 to 64 years old. In a Statistics Canada survey of Canadian nurses, 1.4% of nurses younger than 35 years reported chemical sensitivities, which increased to 3.7% in nurses 35-44 years old, and 4.3% and 4.8% in successive decades. Similarly, in studies in Arizona, 15% of college students and 37% of elderly participants reported heightened sensitivities to chemicals. Increasing prevalence of sensitivities with age is relevant for the aging workforce, as well as for care of the elderly.

## Perception
Unlike perfumes and moulds, electromagnetic fields are usually not perceived. Similarly, electromagnetic sensitivities, while plausible, are poorly recognized. The prevalence of electromagnetic sensitivities is estimated to be 1-3% of the population in various countries.

# Summary
Environmental sensitivities may contribute to a variety of conditions that have been associated with circumstances (e.g. sick buildings), populations (e.g. veterans), chronic symptoms (e.g. pain or fatigue) or initiating/triggering factors (e.g. "chemical injury" or "radiowave sickness"). Criteria established for diagnosis of chemical sensitivities provide a framework for physicians and researchers to examine environmental sensitivities related to chemicals. Although it is not as extensively studied, this pattern may also apply to sensitivities related to electromagnetic phenomena.
Studies to determine the proportion of the population that experiences varying degrees of environmental sensitivities rely upon identification of the condition. Diagnostic criteria would be reflected in the recently-reported prevalence of doctor-diagnosed sensitivities (approximately 1 million Canadians). Evidence is that up to a third of the population may be experiencing discomfort. Environmental sensitivities affect approximately twice as many women as men, and increase with age. Sensitivities may be associated with higher-risk occupations and they disproportionately affect the poor, while the wealthy are more likely to be treated.

# Notes
- (1) Incitant: a factor in the environment that provokes symptoms
- (2) N designates the number of people included in a research study