For me it's not a question of cumulative scent because it's an allergic reaction, not an emotional/psychological reaction. So if I'm surrounded by people who've soaked themselves in eucalyptus oil, which I'm not allergic to, I won't have a reaction. But if I get too close to one person wearing a small amount of cologne that I am allergic to, I might react.
That said, if I'm already sensitized by one person who's wearing a lot of perfume, I will react more to other triggers.
I do try to mitigate my reactions as much as I can, both because it's polite and because I kind of have to in order to live in a scented world. The important thing is that I get to make the calculation of how much hassle I'm willing to go through to be around a perfume-using person. I have one friend who douses himself in cologne--he'll come visit my house, leave, my partner comes home two hours later, sniffs the air, and says, "Oh, was [name] here?" He tries to remember to leave it off before visiting me, but all his clothes are saturated with residual scent. But I still want to spend time with him, so I take a lot of Claritin when I know I'm going to see him. If he comes over to my place, I'll cover a chair before he sits in it and then wash the cover afterwards. I'll drink tea with honey to ease my sore throat. I wouldn't go through this for a lot of people, but for this friend I will. :) Similarly, if I'm going to a big social event that I really want to be at, I'll try to remember to medicate beforehand so that I'm more braced for inevitable scent exposure.
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That said, if I'm already sensitized by one person who's wearing a lot of perfume, I will react more to other triggers.
I do try to mitigate my reactions as much as I can, both because it's polite and because I kind of have to in order to live in a scented world. The important thing is that I get to make the calculation of how much hassle I'm willing to go through to be around a perfume-using person. I have one friend who douses himself in cologne--he'll come visit my house, leave, my partner comes home two hours later, sniffs the air, and says, "Oh, was [name] here?" He tries to remember to leave it off before visiting me, but all his clothes are saturated with residual scent. But I still want to spend time with him, so I take a lot of Claritin when I know I'm going to see him. If he comes over to my place, I'll cover a chair before he sits in it and then wash the cover afterwards. I'll drink tea with honey to ease my sore throat. I wouldn't go through this for a lot of people, but for this friend I will. :) Similarly, if I'm going to a big social event that I really want to be at, I'll try to remember to medicate beforehand so that I'm more braced for inevitable scent exposure.