It sounds like yet another 'de-wormer' (anti-parasitic). You said the following:

"I let my guard down, got into a situation where I should have wore a mask, ..."

I'm a certified scuba diver of over 35 years. Safety is always paramount in diving. Just as a sky diver assures his parachute is every bit correct, so is a scuba diver with his gear. Most people have heard about the "bends" and understand it from nitrogen forced into the blood stream as a result of underwater depth and the pressure associated it. The occurrence of the bends is less frequent than other diving hazards. CO2 toxicity is a danger that scuba divers are well aware of that doesn't really get a lot of mention. If your exhale isn't properly exhausted or the scuba tank is contaminated with 'bad' air such as CO2, the effects of breathing this can be deadly underwater. With depths of one of more atms, it only accelerates and magnified the deleterious effects on the heart, liver, and brain. This is the immediate short-term effect. The long term effect is acidosis, which is always the condition for cancer to spread.

The face mask is precisely the same, but the damage occurs at a slower rate. Hypocapnia and hypoxia are a deadly combination that will lead to acidosis. Even OSHA provides warnings on wearing a face mask. Take note that acidosis isn't a switch that gets tripped, its more a slide bar that goes from small body changes to large scale. It is even accumulate because body chemistry is altered.