For some careful English speakers, nauseous means causing nausea, and nauseated is the term for experiencing nausea. These are the traditional meanings (though nauseous initially meant inclined to nausea before gaining the sense we now consider traditional), and they’re still the ones put forth by some English reference books and usage authorities. In actual usage, though, nauseous has supplanted nauseated in the experiencing nausea sense, and nauseated is reserved for a few specific uses. Nauseating is now the preferred word for causing nausea. The clunky phrasal adjective nausea-inducing is also a popular choice.
English usage sticklers will probably continue to fight the use of nauseous in place of nauseated, but it’s a lost cause. Most edited publications have given up on the old sense of nauseous, and it’s difficult to find more than a few scattered examples of the word used in its old sense in this century.
Nauseating frequently modifies nouns having to do with smell—nauseating odor, nauseating smell, nauseating stench, and so on. To try to get a sense of when nauseating supplanted nauseous in the causing nausea sense, we ran the following ngram graphing the use of the phrases nauseous smell and nauseating smell in a large number of books and periodicals published between 1850 and 2000:
This suggests that the use of nauseated to mean causing nausea gained steam in the early 20th century and was well established by midcentury. Corresponding ngrams charting the use of the phrases only in British books and only in American books look roughly the same. Some English reference books claim the new meaning of nauseous was originally American. Whether or not this is the case, it appears that the adoption of the new sense of nauseous happened rapidly and at roughly the same time throughout the English-speaking world.
It’s still possible to find a few instances of nauseous in its older sense—for example:
What I find particularly nauseous is that Fry is classically educated. [comment on Scotsman]
[I]t is your obligation, when a nauseous odour of none-too-mysterious origins starts to tickle your nostril hairs, to uncork your inhibitions. [Globe and Mail]
But nauseous is far more often used to mean experiencing nausea, as in these cases:
She was nauseous and aching at times, losing weight and feeling so fatigued she could not move. [New York Times]
She has described seeing a woman pleading to be released because she felt nauseous. [Guardian]
For the adjective meaning causing nausea, nauseating, as used below, is preferred over nauseous: