在这个句子中,主干为“it has been found that...”,it作为形式主语,真正指代的是that 引导的主语从句;而在这个主语从句,句子的主干“people can suddenly become sensitive to it”, 且“insensitive to a certain smell”为people 的后置定语;而后面的部分为由when引导的条件状语从句相当于if/ in case引导的条件状语从句;此时,该句的语境表达了“起初对某种特定气味不敏感的人也会突然变得敏感起来,当他们经常……在这种气味中时”。将这四个词代入句子中,exposed一词的汉语意思最符合语境。
However, it has been found that even people insensitive to a certain smell at first can suddenly become sensitive to it whenexposed to it often enough.