Answered by Shaykh Amjad Rasheed
Is it permissible to sniff snuff to bring on a sneeze and ease one’s breathing?
If the thing he is sniffing is only a smell, it does not invalidate the fast. But if the meaning of snuff is what is commonly meant, such that it resembles a powder that one sniffs and it reaches his body cavity, then his fast is invalidated. This is what is said by the vast majority (jumhur) of scholars, as Imam al-Nawawi indicated in his Majmu`.
Amjad Rasheed
[Translated by Sr. Shazia Ahmad]