Gas

Germans with gas masks, First World War.

Gas was very common in WWI yet it seldom gets used in wargames.

Just include a few guys with gasmasks to indicate your troops are wearing them.

Troops that are wearing gas masks get a negative to hit factor to simulate the problem of the small goggles and uncomfortableness of the mask.

Troops who are in gas suffer casualties each turn they are in the gas cloud.  I use cotton balls in yellow and orange for non-persistent and persistent gas.