Apply this rule to drowning or failed swimming rolls, and use the same procedure for strangulation or to dramatize the effect of a cloud of gas containing no oxygen.
If the investigator is unable to breathe, the player attempts a D100 roll of CON x10 or less during the first combat round. In the second round, the roll lowers to CON x9. In the third round, the roll becomes CON x8, and so on, until reaching CON x1. It stays at that multiplier thereafter.
A surprised character has had no time to prepare by inhaling, so the keeper chooses a lower multiplier with which to begin. CON x6 is often appropriate.
If a roll fails, the character has inhaled a medium which cannot be breathed or has begun to suffer serious injury to the respiratory system or brain. He or she loses 1D6 hit points. In each round following, the victim automatically loses another 1D6 hit points. Continue the rolls until escape, rescue or death.