Certain substances are certainly clear cut.

But consider, I dunno, salt? A little built of salt in the environment is normal. But a whole lot of it is pollution.

I’m no climate scientist but I would think it’s similar to toxicity/poisoning. You can die from water intoxication but certainly you need it to live as well. So no, I don’t think it’s subjective. It seems to me that if something is capable of causing harm in a certain amount, it’s poisonous/pollution/toxic.

Yes, the dangers of Dihydrogen monoxide are well known yet underestimated

Dihydrogen monoxide levels have also peaked.

That's more a quibble over dosage, though, than any subjective disagreement on "can too much salt be a pollutant?"