It's genuinely awful.
However, I have two large DIY air filters plus two IKEA filters running and they have made a huge difference this year on the days where air can be cut with a knife.
Still, being unable to ventilate poses problems for shedding heat is only part of the problem. I am too allergic to critical cleaning products to use with closed doors/windows, and even avoiding all that I can still end up reactive to something unknown accumulating in the air. 😐