Making an object collide but transparent

Hello,

I am making a plat-former 2D game where a character runs into a cloud of gas it deducts health points. I have the collision working but in order to make it work the gas has to have a Rigidbody 2D and when my player runs into the gas it moves away from him like it has mass. The lowest you can set the mass too on the Rigidbody is 0.0001. Any help or suggestions would be greatly appreciated.

Thanks, SwampMan