Today was not too bad.
It probably depends on the level of exposure and the type of substance.
But, after my neighbors decided to try the new fabric softener I had to use my respirator and it took me a few days to recover.
The next time it happened I recognized the smell and had my son close all the windows, so it was a less severe reaction.
A fan, actually does the exact opposite, it blows it in my face.

So, the answer is probably that the time of exposure is important. If I recognize it soon enough the reaction is probably less severe. Also, I think it depends on the fluctuations of this illness ( A more severe reaction during less good periods, but I am not sure what is causing what).
I don't think it is a 1:1 relation (meaning that my illness or even more severe exacerbation are fully explained by this), but I think it does have a significant contribution.

When I worked in a pathology lab 15 years ago, I developed a fairly severe hypersensitivity reaction to formaldehyde. But, this fully resolved once I stopped working there. Some years later, I developed an allergic reaction to latex (which is not unusual for health care workers) and then hypersensitivity to DMSO (this coincided with my MG symptoms). At a much later stage, I realized that formaldehyde is in many household items which I used regularly.

I don't know if this is applicable to every MG patient, but I know some do notice worsening MG symptoms after being exposed to certain substances. I don't know what the exact mechanism would be.