Yellowing on old computers, especially ABS plastic, is usually due to UV light, but maybe cigarettes do it too.
The sunlight here in Tasmania is absolutely hectic – half my computers look like characters from the simpsons! It doesn’t even need to be falling on it directly and the plastic curdles like milk.
There is a treatment to reverse it called retrobright though. I’ve had mixed results, but it might be worth looking into.
