It's not one spectrum from "too clean" to "too dirty". It's at the very least two. For example, let's say pollen and coal. What you're talking about, having al

It's not one spectrum from "too clean" to "too dirty". It's at the very least two. For example, let's say pollen and coal. What you're talking about, having allergy issues from too little exposure, applies to pollen. Let's call those "adaptive particles" (term I just made up). What you're talking about does not apply to coal. The ideal amount of coal particles in your lungs is zero. There is no age where it helps to get exposed, no allergy to avoid getting. Let's call these "non-adaptive particles". If you live in a city, it's very likely that the negative impact from these non-adaptive is way worse than the negative impact of missing out on the adaptive particles. Breathing the air, you're increasing your risk of asthma a lot to decrease your chance of allergies a bit.