Heavy particles and gaseous emissions are not comparable in such a simplistic way. If you take a dump on the street it doesn't mean you caused 50 million times more emissions than the EPA limits for ICE car exhaust.
For example, iron from brakes is heavy but ecologically pretty harmless. OTOH NO₂ weighs almost nothing, but is toxic. You can eat 30mg of iron per day to stay healthy (just don't lick it off the asphalt directly), but a similar amount of NO₂ would be lethal.
Heavy particles don't stay in the air for long, and don't get easily absorbed into organisms. OTOH gaseous emissions and small particulates from combustion can linger in the air, and can get absorbed into the lungs and the bloodsteam.
Yeah, but brakes are not not made from pure iron and you won't have atomic erosion. Silly argument, really. Notoriously, you could still find brake pads with asbestos not too long ago. Pretty much any fine dust is very unhealthy to inhale, but brakes and tires are made from material mixes you really don't want to breath in. Even the "inert" fraction we find as microplastics in everything, the rain, fish and newborn, and we're only beginning to understand their biological reactivity and long term health consequences.
the break pad and tire particles in question are not so large they precipitate immediately. They aren't iron but rather real/synthetic rubber and other organics. There is research on them being bad for human health.
For example, iron from brakes is heavy but ecologically pretty harmless. OTOH NO₂ weighs almost nothing, but is toxic. You can eat 30mg of iron per day to stay healthy (just don't lick it off the asphalt directly), but a similar amount of NO₂ would be lethal.
Heavy particles don't stay in the air for long, and don't get easily absorbed into organisms. OTOH gaseous emissions and small particulates from combustion can linger in the air, and can get absorbed into the lungs and the bloodsteam.