Nerve Gas = Does high damage, when in contact. Health damage stops soon after you get out of the gas (you cough for very short time). Because the effect of the weapon is comparably weaker when after an enemy steps out of your gas... it seems to work best when thrown into a tight space in pairs. In a hallway, two nerve gas nades go off, and you try to run through them both, you die. So Nerve Gas.. get out fast, but not by running through it.
Incendiary = Low damage in contact, Low damage after contact, but lasts longer than nerve gas (because you catch on fire). So unlike the nerve gas, when you are hit with the Incendiary Nade.. the chances are, you wont die from it. If you run through two of the fire nade, it almost takes the same amount as running through one fire nade. And once u catch on fire, your health will decrease gradually.
So my tactic is immediately get out from nerve gas. But for fire nade, i avoid it of course.. but if I'm ALREADY on fire.. then I just ignore it and try to get as many kills as possible, inside or outside of the flames. Because the fire on me will eventually kill me if I am low on health, no matter what I do. And if I have high health.. then I just think of it as just getting shot on leg twice.