Jenny.. were you perhaps thinking about carbon dioxide, or carbon monoxide? The first would suffocate you by causing lack of oxygen, the second would actually poison you. I don't think either would trigger allergic reaction, they would just lead to death if in sufficient levels.
Ok - my friend's dog has been having problems with ear and eye infections and a cough in the past and vet has been treating with whatever creams etc but they keep recurring.
However friend has been in hospital again and dog went into kennels for about 5 days and when she came back when friend got out - she bounced into the door like a 2 year old and nothing wrong with her.
Within two days all the symptoms came back and she rang vet (who wasn't very helpful) and asked could the oxygen be the cause. she also rang the kennels and they said she had been brilliant when there so the only thing "new" in her house is the oxygen. That is why I asked. She now thinks it is an allergy.
I know - this friend has animals all her life cats and dogs and they were all ok but poor doggy hasn't been well from the oxygen came in so she was trying to put two and two together.