There's a correlation between old school values and avoiding new school problems.

Electronics and tightly sealed houses, both of which the Amish might avoid, together allow for a lot more formaldehyde build up indoors. My pet theory (with no proven evidence except my own two eyes, as someone possibly affected by formaldehyde, which means the details are just guesswork right now) is that formaldehyde indoors is responsible for the increase in reported allergies, poor vision (glasses), asthma, ADHD, and possibly increases in divorce rates or staying single - by which I mean that it can cause irritability.

I figure it is the primary cause of road rage, that it can possibly bind to and release microparticulate of metals like iron and aluminum, that it can store itself not just as a solid at room temperature but also in the rubber parts of a scooter while it charges or silicone or foam parts of a CPAP as you breathe in and out (you naturally produce formaldehyde, but increased presence in your exhaled breath has been associated with cancer, for example).

It also causes insomnia and can cause very low humidity in an enclosed space, which might both increase static shocks but also possibly break electronics when combined with its effect on certain metals mentioned earlier.

I’ve an even crazier pet theory that in the presence of other VOCs and sunlight, formaldehyde can multiply, but I don’t have anything to back that up. Formaldehyde with CO2 and UVA can react to become ozone, but ozone with UVA and other VOCs can become formaldehyde. As a result, on a particularly sunny day, I think even outdoor formaldehyde levels can rise and cause the day to feel even warmer than it otherwise should, and that it’s the formaldehyde that can then cause more inattentive accidents.

I’ve another theory that if you take something on to a train with micro metal particulate offgassing and formaldehyde, that it will bind itself to the heat of the wheels over the tracks and be released along with microparticulate from the metal rails every time the train runs by.

I could give more evidence of why this might be so, such as increased rates of emergency repairs of train tracks in my area, Toronto, and a study from 2017 that says Torontos subways have the most metal particulate in NA, but since it’s just speculation right now, take everything I said with a grain of salt, please.

I should add that burning natural gas indoors without appropriate airflow is a wonderful way to introduce a lot of formaldehyde to your living quarters over the years. If I could ban all forms of combustion indoors, I would, I really would.

So it seems like exposure to microbial loads from farm animals help prevent allergies. But humans have not always kept lifestock - what kind of microbial exposure do pre-agrarian peoples receive?