Things are simultaneously heating up and cooling down
Comments
Most people with quirks are able to handle the effects, at least to an extent- like Endeavor with his flames, Bakugo and his explosions, and Kaminari with his electricity, etc, but there are limits, and it seems like they can train to raise them. As far as Dabi/Toya, we saw in a flashback/backstory scene last season a doctor talking to Endeavor and his wife, saying that while Toya has a strong fire quirk, his body took after his mother, and is better suited for withstanding cold- and they were told to not let Toya use his quirk- and Endeavor could always tell that he had been using the quirk because he would have new injuries/burns on his body.