When a gaseous status effecting spell or action (poison, sleep, paralyze, etc.) is used within 25 ft of the caster, use a reaction for the shield to absorb the gas from the air. 

When a liquid status effecting spell or action comes into contact with the shield use a reaction to absorbed it.

After a toxin has been Absorbed by the shield a reaction can be taken on subsequent turns, when an enemy makes an attack against the caster they may block with the shield, momentarily increasing their AC by 2 . And on a successful block they cast Burst, which releases the Absorbed toxin in a 15ft radius around the enemy.