Yes, fossil fuels are the best to keep pollution away, just need to installed perfectly, configured and maintained regularly, monitored to make sure everything is running correctly, and have additional properties lying around vacant just in case there are leaks, misconfigurations, poor installation, etc. But we must use fossil fuels, there are no other options!

Could also just be the furnace. Incomplete combustion conditions can give rise to the symptoms mentioned here.

I doubt it's the furnace, unless there's a serial defect/recall.

The problem is likely "between the keyboard and the chair." ;-)

I had a gas furnace that wasn't properly maintained as far as cleaning. Result: insufficient air flow for full combustion. Secondary result: CO build up in basement space. Tertiary result: asthma-like symptoms for me.

I believe it.