In particular[0], the bigger stuff (water) blocks the smaller. > Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humi

In particular[0], the bigger stuff (water) blocks the smaller. > Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water. So in the absence of the water, the He would escape. [0] heh