A species of termites emits a very small concentration of naphthalene to suppress fungus in its mounds and galleries. It was (willing suspension of disbelief) amplified to provide drama in an episode of Fox network's House.

Methane emission from anerobic gut fermentation (termites, cows) is not toxic as such. In confined spaces it may displace oxygen and it is flammable.

Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf