From a public health standpoint, infectious diseases dominated the 19th century. Then during the 20th century, cancer was the main concern. At the beginning of the 21st century, public health officials are looking increasingly at chronic diseases and illnesses that are closely associated with an increasingly polluted environment. These conditions include environmental sensitivities, but also heart and lung diseases, allergies and food intolerances, learning disabilities in children, fertility problems, as well as other emerging illnesses such as fibromyalgia and chronic fatigue syndrome.
In 2003, according to the National Population Health Survey conducted by Statistics Canada, between 2 % to 3 % of the population (depending on the age) stated they had been diagnosed with multiple chemical sensitivities by a health care practitioner. Although environmental sensitivities may occur at any age, it would seem that the number of cases increases with age and that the condition affects women three times more than men (Table 2). There is every reason to believe that the proportions are similar in the Quebec population.
| % of the pop | % of women | % of men | |
| 12 years old and older | 2,2 | 3,1 | 1,2 |
| 12 to 24 | 0,8 | 1,0 | 0,6 |
| 25 to 39 | 1,6 | 2,0 | 1,1 |
| 40 to 49 | 2,7 | 3,7 | 1,5 |
| 50 to 59 | 3,5 | 5,2 | 1,7 |
| 60 to 64 | 3,6 | 5,8 | 1,4 |
| 65 to 69 | 3,3 | 4,6 | 1,8 |
| 70 to 79 | 3,1 | 4,2 | 1,8 |
| 80 and older | 1,9 | 2,3 | 1,1 |
| Source : National Population Health Survey, Statistics Canada, 2007. | |||
Since these numbers reflect only those who went to a “health care professional” due to their symptoms, the number of affected people with environmental sensitivities could be much higher. Different studies in the United States have revealed that between 11% to 33% of those surveyed stated they had heightened sensitivities to common household chemical products. As for sensitivities to electroma- gnetic phenomena, it is estimated that it affects between 1.5 % to 8% of the population.
Environmental sensitivities affect all socio-economic groups. However, people from lower socio-economic groups are more susceptible to suffer from them. It should be noted that, in general, people with lower incomes are more likely to be exposed to environmental contaminants and to stress factors related to their unstable financial situation.