Home and personal cleanliness
If home and personal cleanliness contributes per se, its role is likely to be small relative to the other factors.
Even the cleanest-looking homes are full of bacteria, viruses, fungi, etc.
Routine weekly cleaning has no sustained effect on microbe levels
We cannot create a “sterile” home - microbes are constantly replaced via dust, air, body flora, pets, contaminated foods etc
A key point may be that the microbial content of modern homes has altered because mostly our homes now interact with urban environments. These lack the diverse range of microbes found in rural environments and are populated by people with different and less diverse human microbiome.
This means we now interact with an altogether different and less diverse mix of microbes relative to earlier generations which were largely rural.
Helminths (worm) infections are now relatively rare in UK - up to 1950s , 50% children were infested with pinworm
What about personal cleanliness?
In his early proposition Strachan suggested that, “higher standards of personal cleanliness” could also be an underlying cause of reduced microbe exposure
The entire skin surface has about one million resident bacteria per cm2; numbers vary from 1,000 per cm2 on forearms to millions per cm2 on underarms.
Our habit of bathing/showering, shampooing have increased since 1940s and 50s. Bathing and showering removes many microbes from skin but are rapidly replaced
Whether, or to what extent, skin microbiota might contribute to the OF mechanism has not yet been studied.