Tear gas and pepper spray 101, from Occupy Oakland Medic Collective. USA-centric, but still relevant to lots of other countries.
Tear gas and medical info for protesters, by Starhawk. Quite detailed, goes into respirators and stocking a protest first aid kit.
Who should avoid tear gas, by the Black Cross Health Collective. Contains useful health warnings, and also a bit more on remedies.
I think it’s interesting that one of these links is from Starhawk . Some comments on the original thread I grabbed this from dismissed the suggestion of Rescue Remedy. For anyone who doesn’t know what Rescue Remedy is, it’s a homeopathic blend of diluted Impatiens, Star of Bethlehem, Cherry Plum, Rock Rose, and Clematis . These flowers are thought to have “vibrational energies” that are useful for when people need calming or have had a shock to their system, both physically and emotionally. Personally, I’m not a huge believer in homeopathy but I know people who swear by Rescue Remedy and always have some on hand.