I wonder why we focus so much on this claim, when there are many studies giving other plausible explanations.

> Living less than 75 m from the main road was significantly associated with lifetime allergic rhinitis (AR), past-year AR symptoms, diagnosed AR, and treated AR. The distance to the main road (P for trend=0.001), the length of the main road (P for trend=0.041), and the proportion of the main road area (P for trend=0.006) had an exposure-response relationship with allergic sensitization. A strong inverse association was observed between residential proximity to the main load and lung function, especially FEV1, FEV1/FVC, and FEF25-75.

Effect of Traffic-Related Air Pollution on Allergic Disease: Results of the Children's Health and Environmental Research - PMC - https://pmc.ncbi.nlm.nih.gov/articles/PMC4446634/

> The most serious issue might be the growing trend in sensitization to pollen, especially in urban settings (7, 8); in fact, people living near heavy traffic are affected with pollen-induced respiratory allergies more than those in rural districts (9). The sudden rise in environmental pollutant levels due to industrial development and urban motor vehicle traffic has affected air quality and consequently, the severity and mortality from allergic diseases (10). Some evidence suggests that air pollution might cause new cases of asthma as well (9, 11).

Interaction Between Air Pollutants and Pollen Grains: The Role on the Rising Trend in Allergy - https://pmc.ncbi.nlm.nih.gov/articles/PMC5941124/

This doesn't mean that exposure to biodiversity doesn't play a role, but when it comes to explaining the differences between rural and urban settings, this explanation seems more plausible to me than the hand-wavey claims about people supposedly cleaning their apartments more in cities.

Personally, I have seasonal asthma associated with pollen, since childhood, and I'm from a big city.

I have a much harder time walking next to a busy road in allergy season than being somewhere more rural, even when there are birch trees right in the vicinity of where I am, one of my allergenes.

It's not b/w of course though, the pollen can trigger it not only in the city. But then it's usually very mild.

My asthma is seasonal, allergy-associated, and still, the worst stressor I experience is pollution and car exhaust. Well, the worst unavoidable stressor.

Alcohol also seems to do bad things to my allergy response.

Japan is gripped by mass allergies. A 1950s project is to blame