I have always assumed that by “environmental factors”, they were talking about, primarily: infections.
It has to be an infection, or infections, that are quite common “in the wild”, as if it was rare, we wouldn’t see so many people with MS. But it must also be something that does NOT have serious health implications for most people who come into contact with it - otherwise we’d be able to trace it easily!
So in short, it has to be something lots of people get, but not many have long-term complications.
I think smoking is one of the very few environmental factors that has been shown to influence both risk of MS, and its subsequent progression. But having said that, I’ve never been a smoker, and never been exposed to significant cigarette smoke as a child, or at any period in my life. So if smoking (active or passive) is a cause, it certainly can’t be the only one, because plenty of MSers can’t tick the box.
The same with allergies. Millions suffer from allergies. Only a tiny fraction go on to get MS. Also people with NO reported allergies still get MS.
So it can’t be that, either.
Years ago (this a bit of an off-topic ramble now, and might not be at all relevant), I saw a TV programme, that was something to do with the immune system, and one of the tests it showed was to scratch your own skin quite lightly with a fingernail, and leave 15 minutes, and see if a red welt develops.
Well, being a kid, I tried this, and it DID! So I showed my parents, and said: “Look, mine did it!”, but all they said was: “Oh, don’t do that to yourself!”
I’ve no idea what the test was indicative of, now, or even whether it was at all scientific. But I do remember thinking mine was positive, and that it meant something.
Now if only I could remember what…