Ionisation smoke alarms triggered by steam?

I have three BRK 670MBX ionisation smoke alarms.

The the ground floor and 1st floor ones go off frequently after cooking in the kitchen or using the shower, apparently triggered by steam / humid air.

I reckon this is not normal, they should only be triggered by smoke?

The product has good reviews, so I'm wondering what's going on here. Is this expected behaviour or are they faulty?

They're 3.5 years old, at the beginning it didn't happen, but then the kitchen and shower may have been used differently (I had a low-flow shower head, for example...)

BRK are a reputable manufacturer
Ionisation smoke detectors are designed to pick up particles within the chamber of the smoke detector. These particles could be from cooking or a shower.
Use a vacum cleaner and clean the smoke detector by holding the nozzle close to the mess chamber.This may help.

Thanks, I've vacuum cleaned it, but only from the outside, i.e. by holding the nozzle to the grille, without taking the detachable unit off. Hope this helps.

If it doesn't, I'll take it off and give it a more thorough vacuum clean.

Get a can of compressed air and give it a good blast out !