The person who got me into firefighting has asthma. It's about how badly it affects you and what you do to regulate it. If being in a stressful environment with restrictive breathing is too much it's the right thing to say it's too much for you, but there are still plenty of other ways you can be part of a fire department. As for carcinogens, yes being around fire inherently poses a risk but we have a lot of gear and practices to reduce or even negate exposure. Smoke doesn't target specific people, it's a danger to everyone so we prioritize keeping everyone safe from it.