From what I've understood on the subject is that it's not super-well understood yet, with the common advice early on having been to avoid exposure to common allergens for young children (because potential anaphylactic shock being a bad thing), until it was observed that children i Israel generally showing a lot lower rates of peanut allergies, and that very young children in Israel are exposed to peanut-containing snacks early on. At this point, the advice was reversed to that young children should be exposed to potential allergens early on (because developing a life-long allergy from under-exposure to an allergen is worse).

The rest I'm guessing is extrapolation from that - lack of early exposure = negative.

That is how I’ve understood it: residues of bactericides everywhere which become inhaled dust.