Q: I have recently started seeing someone I really enjoy. The only problem is she uses a lot of synthetic fragrances (body wash, shampoo, laundry detergent, perfume) that all make me feel a bit sick.
I only use natural stuff and would like her to, too. I have tried asking (as tactfully as possible) a former lover as well as former roommates to change their products, even offering to subsidize any increased cost, and have never gotten a positive response. I'm afraid to bring this up because I don't want to turn this into a fight, but it's not something I can ignore indefinitely.
A: It's possible your tact is the problem. What you have isn't an opinion, it's a physical reaction.
So present it that way: "Perfumes make me sick." Pad the preface as you want ("This might sound weird/intrusive/(etc.)"), but be completely plain with the point.
Then, specifics: "I'm sensitive to anything with a synthetic fragrance — shampoo, detergent, scented candles." The last one is unrelated to her body, which might help.
Then, ask: "Would you be willing to change what you use?"
For someone I see often, I'd sign on without thinking twice. The right person for you will do the same. Obstacles like these are a natural filter; trust that the keepers will pass right through.