I always thought that was strange. Everyone aboard the Normandy undergoes decon whenever they board the ship. It's pretty much as sterile an environment as the Flotilla is.
They actually explained it a bit more in several of Tali's quest chains and some of her little skits. The air is sterilized 'enough' for the races. But not enough for Tali's people. Seriously. A single speck of dust seems to be capable of screwing with their immune systems. The Flotilla actually has 'the most advanced filters in the known universe' which is actually said in-game.
And yet they're nearly all scientists and wanderers. Seems a bit ironic that the race that needs the most particular environment is the one that travels through inhospitable ones the most.
Honestly, I forget if they need something extra in their air or not. We didn't get told all that much about their planet the Geth destroyed. But I do know that they can take their suits off in special stations their race lives in out in space.
Heck, we don't really know what Tali and her race look like. Other than doctored photos from fans, the only image in-game that exists was a bullshit stock photo that pissed off gamers immensely. Look it up. Its a photo pulled off some website of a woman in a sweater or shawl. Talk about lazy, lol.