Smoke inhalation rules

My PCs might end up trapped in a buring building, I was looking to show the effects of smoke before the fire gets to them.

Each turn a charter is trapped in the smoke they must take a Toughness getting progressive harder as the smoke builds up. Round 1 at +20 round 2 at +0 round 3 at -10 etc.

If the test is failed that charter takes wound equal to the degrees of failure.