Tear gas (or CS) can be dealt with, as a good gas mask can keep you fully operational in a gas environment.
For those who have not been to a gas chamber, tear gas can be dangerous in large quantities, as the gas can be ignited to form a ball of flame or explosion. It also contains arsenic. This was used in large quantities by Janet Reno's US army force and FBI agents when they attacked the women and children of the Branch Davidian Compound in Waco TX. The massive amounts of arsenic breathed by these people in the tear gas, resulted in their muscles contracting so strongly throughout their bodies that they snapped their own bones before dying. This happened indoors, and is less likely to happen out of doors.
Tear gas is only an irritant to the skin.
The eyes and lungs are affected immediately, effectively incapacitating the victim who stays in the gas.
There are various gas mask designs, such as those that are essentially a rubber bag over the head that has elastic around the neck, and those nice full faced view designs used for hazardous work environments by firefighters and others. This type can be expensive.
The best military designs strap to the head and seal to the face. This is efficient, sealing the eyes, nose and mouth, is easier to be aware of a seal compromise, and gives you less to worry about when moving.
Whatever design you choose, it should have a new filter and should be the right size for your head. Many gas masks have lenses that are very difficult to see out of, so keep that in mind when you are looking at surplus masks.
This german gas mask and filter are a good buy. Cannot be found cheaper. They are new, high quality, German made, sealed-to-full-face design with a new NATO charcoal filter. They have broad lenses for field-of-view. You can wear prescription glasses inside the mask.
I only searched for this bargain. I am not selling the product. But it really is a very good bargain at $20.