I think it's easiest to get negative modifiers for the comfort of living through radiation:
1. Flat plains city sites are the most exposed to radiation.
2. High-ground city sites are exposed to radiation more than regular ones.
3. Nuking a pole raises the climate zone's radiation permanently. It also increases global radiation levels, but only for several turns, so time it right. You can also nuke both poles and stack modifiers.
4. Sometimes, weekly challenges have "helpful" modifiers like negative global comfort of living, so check them out occasionally.