A set of pads typically lasts about as long as a set of tires (at the very least they're within a factor of two of each other). Compare relative volumes of materials. Brake dust is always going to be a much smaller source of particulate matter in the air than tire dust.
FWIW if the fumes/particulate from burning them are any indication tires are probably way nastier than brakes.
Second paragraph, though: I've never been around a brake pad fire, but I'd rather breathe vulcanized rubber than "metal shavings of copper, steel, graphite, and brass bonded with resin":
In my non-chemist, non-doctor opinion the circumstantial evidence points to brake dust being way less bad than tire dust.
Being inert in the presence of most common substances tends to correlate well with low toxicity. It's the stuff that chemically reacts to all sorts of things that tends to be the most poisonous.
Tires break down over time under "normal earthly conditions" which includes a lot of the same temperatures and molecules that are found inside the human body. To me this is a massive red flag for toxicity.
High temperature tends to make things react more readily. Most of the stuff in brakes has to be really stable at high temperatures and being stable at high temperatures tends to correlate well with being chemically inert.
The most reactive stuff in brakes are the long complex molecules used to bind it all together. These sorts of molecules are similiar to the molecules used in tires and all sorts of other plastic like things. So all else being equal tires probably have more of the chemically interesting things that cause problems in the human body.
Burning brakes smells way better than burning tires which seems to confirm this (remember, your nose has millions of years of incremental improvement when it comes to indicating what is and isn't suitable to ingest).
And before anyone thinks they're gonna get some easy internet points by saying "but asbestos", the amount of asbestos in brake pads is nonzero (to account for the sketchy unbranded stuff that may or may not use it) but still tiny enough that people working in proximity to it have mostly stopped dropping dead at 60 thanks to lung cancer so I think that however much asbestos is in brake pads it's low enough to not be an issue from a public health POV.
TL;DR I'll take my chances with brakes over tires.
Would have guessed the opposite, FWIW, on the grounds that the bad stuff is hard materials (like mine dust). But if anyone actually knows that would be interesting. How good a measure is total PM2.5 or whatever, without asking about what it's made of?
That said, as was pointed out below, changing the car's design to vacuum up brake dust would be much easier than getting away from rubber.
Yeah, you generally shouldn't be inhaling dust but inert dust (silica -> silicosis) is better than reactive dust (asbestos -> cancer).
I'm not well versed enough on the specifics to tell you whether or not PM2.5 is a good measure of pollution. The experts in the subject of air pollution routinely use it so it can't be thaaaat bad.
> Compare relative volumes of materials. Brake dust is always going to be a much smaller source of particulate matter in the air than tire dust.
It's not a fair comparison, because you don't wear through the entire tire material as extensively as a brake pad. Fortunately we don't have to back of the napkin this one, there's lots of research showing brakes are near peers or exceed tires in contribution to PM 2.5.
As to your second point that brake material might be less harmful -- there is some initial research on how composition mediates impact.
Aluminum, sulfate, nickel, arsenic, and silicon seem to have the strongest relations. Aluminum is a frequent abrasive in brake pads, they aren't merely contributing some form of benign 2.5.
I can't do a comparison, you might be right it's "less bad" than tires, which contain other heavy metals. But I do not think the research is powerful enough to precisely settle that question beyond the overwhelming academic consensus that both are bad and we should definitely try to reduce both.
FWIW if the fumes/particulate from burning them are any indication tires are probably way nastier than brakes.