X-Men: Watchers on the Walls, Christopher L. Bennett

The X-Men try to defend some helpless refugees from being fired on by another spacecraft. But it turns out that the issue is way more complicated than that. Something in the refugees' genetic makeup turns oxegen into chlorine--something that can't be stopped once it starts. These "chlorites" will destroy the Earth unless they are stopped.

This would be a fascinating novel if the author hadn't insisted on writing like the comic books. As it is, it's interesting--but extremely annoying.