Based on what I see here the general advice is that you need a purpose built half or full face respirator to really have any success against tear gas. However if you aren’t willing to pop for a respirator that costs $100-$200 this Professor recommends a form of disposable respirator, the N100.
For protection against tear gas, Professor Ignatius Yu, editor for the Hong Kong Academy of Medicine, recommends wearing a tight-fitting respirator with an appropriate filter, such as a full-face mask with activated charcoal filters that absorb organic matter in tear gas.
“Tear gas comes in particles as well as gaseous forms, so you need protection from both. The usual type of particle filter [mask] will not be able to protect you,” he says.
…
He suggests another mask, N100, which has a nearly 100 per cent efficiency in filtering airborne particles, and an ability to block smaller particles than the N95.
Most masks offer no protection from pollution or tear gas | South China Morning Post
https://www.amazon.com/3M-Particulate-Respirator-8233-N100/dp/B008MCV43K/ref=sr_1_1?ie=UTF8&qid=1410148165&sr=8-1&keywords=n100+respirator
