Now new research confirms that this is the case, suggesting that people have become more sensitive to environmental allergic assaults over the last 25 years.
Using blood samples provided over time by British men, the study showed that allergies to pollen, pet dander, and other common allergy triggers have increased by nearly 5% per decade since the mid-1970s.
But a researcher on the study says he doesn't know why.
"There seems to be something that is making people more sensitive to these allergens," epidemiologist Malcolm Law, MD, tells WebMD. "It is probably something to do with Western lifestyle, but what it is we don't know."
Law and colleagues analyzed blood samples from 513 middle-aged men attending a medical center in London between 1996 and 1998. They then matched the blood to stored samples taken from 513 men between 1981 and 1982 and to 513 samples taken between 1975 and 1976.
All the samples were tested for sensitivity to 11 allergy triggers including grass and tree pollens, household dust mites, and pet dander. Positive samples were retested for specific immune-allergy responses to inhaled grass and tree pollen, and cat dander.
The researchers reported "highly significant increases" over time in the proportion of men testing positive for the allergy triggers, with specific increases in antibodies to the three inhaled allergens.
No Decline With Age
The average rate of increase was equivalent to an additional 4.5% of men developing allergies each decade. There was also no evidence that allergic reactions declined as the men grew older, as has been reported in other studies.
The findings are reported in the April 15 online edition of the British Medical Journal.
Allergy and asthma specialist Pramod Kelkar, MD, tells WebMD that the strength of the study is its length.
"This study spanned several decades, and we haven't seen that before," he says.
The researchers conclude that the increase in susceptibility is not likely to be due to increased environmental exposures or to a decline in childhood infections in recent decades -- two popular theories for why allergic diseases are on the rise.
"We just don't have the answers," Law says.