Sudden sneezing in cat, when to take to vet?

I have a 2yo male domestic shorthair. Today he has sneezed 16 times, each time in short bursts of 3-4. Sneezes started very loud, have quieted down. This is the first time this has happened. No temperature, no weight loss. Eyes clear. No issues breathing. Nose a little wet, but his nose is usually wet! Has eaten breakfast, dinner, and had five treats (normal eating patterns). I have noticed a bit more sleepiness than usual today (he was playful up til last night). He has a brother in the same household (same age and stats) who is not sneezing. According to my air purifier, air quality in my area is very bad today, so I'm hoping it's dust allergy but feel anxious in case it's a cold. I would usually make an appointment, but the vet stresses him out quite a bit, and being separated stresses his coward brother out even more, so would like to make sure I'm not worrying them both unnecessarily.

Hello and thankyou for this interesting question.  If at any time a cat may need to see a vet, you can call your practise and ask them to triage the situation - ie to quickly go over their condition in order to determine how soon an appointment is needed, and what signs would indicate that you need to get an earlier one.  Indeed, some sneezing cats do have allergies or have been exposed to a high level of particulate matter or dust.  Others may have polyps, foreign bodies or tumous in the nose; others still have a 'cold' or viral infection or changes to the lungs.  Thus sneezing is a symptom, which in some cats might simply be very minor, but may also indicate something serious.  Even in cases where a cat seems very well in themselves, sneezing that has continued for several days merits an appointment.