Heater is making my eyes sting

Not just sting, but burn. I get out of my car looking like I had just cried. I've dealt with an exhaust leak in an older car before and the eye irritation was mild compared to this.

It doesn't seem to be about the fan running in general, but rather only when the heat turns on based around the ambient temperature.

For example, 72 during the day blows cold - no irritation. 72 at night blows warmer air - eyes burn.

So that makes it seem like it's not just a dirty air filter. What could be the problem?