Depends on the concentration in the air. The concentration required to be lethal via inhalation may be vastly less than to be lethal via absorption through the skin, and therefore a mask may be the difference between life and death.

In Soviet Army the gas masks were always used along with OZK (common troop protection kit), which is basically a full body rubber suit. It's reasonable to expect that Russian-drilled, chemical weapons possessing Syrian military has the same procedure. These suits are even cheaper than the gas masks.