Open a window if there are any odd smells. R-22 has a rather distinctive odor.
Not poisonous, but it will exclude oxygen and freeze anything nearby as it vents/boils.
Due to the latter, store it only in rear cargo, tied down securely. Assume you will encounter severe turbulence even if it's really unlikely. If the tank requires upright storage, store it upright and tied in place.
And a 1 atm change in gauge pressure will make very little difference. It's much less than a change in pressure due to storing the cylinder on a hot day vs. a cold day. It's always liquid and gas in equilibrium, so the internal pressure is determined only by temperature (it's just the vapor pressure).
There may be some legalities about hauling it for hire.