The Stench spell, as (very well) re-written in DFRPG Spells, is a pretty lethal spell.
It causes suffocation, and eventually flat-out damage. However, you can hold your breath for up to HT seconds.
Naturally this has caused a need for some rulings:
- when can you choose to hold your breath? Is it a free action on your turn? Is it a defensive small-m maneuver like an Active Defense?
- Is it realistic that you can continue to fight without penalty while holding your breath? Maybe. I've held my breath 10+ seconds while doing deadlifts for higher reps (say, 10-12). But it's not optimal. GURPS doesn't penalize it, either way.
- how long does it take to form the cloud?
Generally what I've ruled is:
- you can choose on your turn. Before then, you're breathing.
- I don't penalize holding your breath when you fight, but I do limit actions. How are you doing Intimidation with a closed mouth? Don't use Tactics to help your buddy. You can't yell out about the flank attack.
- Spells doesn't seem to mention an onset time, but one solution to "Can I hold my breath as a Defense?" or "Am I holding my breath?" is to give it one. Make it take one full second to form up. The caster's next turn determines when it starts, giving everyone a chance to close their mouth and hold on. I actually don't use this, I ruled it forms instantly, but you can decide on the start of your turn if you reacted by holding your breath or not. Everyone says they did. Because of that, I've ruled:
- no in-game discussion and I frown on out-of-game tactical discussion. If you're all in a Stench spell area effect and holding your breath, then how are you getting to, "Are we holding this position or should we back off towards the other room?" Inevitably someone busts out "Oh, uh, I roll Gesture to communicate that." Gesture is helmet taps, holding up a hand for stop, and how I communicate with my fellow drivers in NJ.
That's pretty much how I've handled Stench in play.