If it were always the case that only one pilot suffered these symptoms, I would be deeply concerned.
The sensations of smell, followed by the other debilitating feelings, can be symptomatic of another quite serious malaise. It is the times that there is only one pilot thus affected that worries me.
There needs to be a clear cut assessment of the incidents defining when both pilots have symptoms v one pilot.
Having said this, some people react to smoke etc by shallow breathing. The result during a period of increased heart-rate, i.e. take off phase, will of course be accentuated.
Going onto oxygen is the obvious course of action even if the problem is slight.