Nauseous is often used to mean nauseated, or feeling sick. So a sentence like I feel nauseous is common and widely accepted in modern English.
Older usage advice often preferred a stricter distinction:
- Nauseous meant causing nausea.
- Nauseated meant feeling nausea.
Under that older rule, The smell is nauseous meant the smell makes people feel sick, while I am nauseated meant I feel sick.
In current real world usage, however, many careful speakers and major dictionaries accept nauseous for both meanings. That means both of these are standard in many contexts:
- I feel nauseous.
- The odor is nauseous.
If you want to avoid complaints from traditionalists, use nauseated for a person who feels sick. But if you say nauseous that way, you are still using a form that modern English widely accepts.