Carbon Monoxide poisoning, although difficult to get with modern compressor technology is something divers must be aware of.

CO2 is odorless and tasteless so recognition of signs and symptoms is imperative. 

Red nails and lips are difficult to see while underwater due to light absorption but other symptoms such as dizziness, headache, and vomiting while diving should always prompt the end of a dive and testing of air quality as a precaution if red lips and nails are present once surfaced.