If there was to be a decompression actually while they were smoking, they would merely be reduced to our level.
My father smoked 80 untipped per day and owned and piloted an Airspeed Consul in the 1950s and 60s. He maintained that his heavy smoking gave him a much higher ceiling to non-smokers. The Consul wasn't fitted with oxygen (thank God as he chain-smoked during all phases of flight) and he said that he would often amuse himself flying to the absolute ceiling on long flights. I always though it was a daft thing to do but he lived to tell the tale so maybe there is something in it? The gasoline fumes were strong in the Consul -- very noticeable to a young child such as me -- I reckon now that the brass fuel primers were leaky. Nobody would allow that today, eh?