1995 and older heavy duty diesels are now banned in California (with some exceptions). 1996-2009 will be banned over 2020-2023.

Most trucks on the road should be Tier IV and using DPF + DEF. I think they mandated it as a requirement for all semi trucks a few years back as well.

There are people who install "delete-pipes" though, and they should be taken out back and made to inhale what they spew.

For anyone else curious, I googled it and it looks like a delete-pipe, (aka muffler delete, cat delete) is basicly a muffler/catalytic converter bypass device. Some do it for performance, some for the noise, some explicitly to "piss off tree-hugging liberals".

Good god have you ever see someone "rolling coal"? it's insane.

Big lifted trucks spouting thick black smoke from their exhaust that leave behind little ashy flecks.

Oh yeah, we get plenty of them out here in the rural parts. Friggin stupid.

We've got a little '81 tractor that's a 3-cyc 1.4L diesel and if the wind blows the wrong way you definitely don't want to inhale too deeply. I'd love to convert it to Tier IV but there just aren't kits made for it.

I'm now curious how difficult it would be to cycle with a respirator.

I've seen one person riding with something that looked like a gas mask. It appeared to have a "bane" like square filter near the mouth. Not just a simple N95 mask.

some people use these for low oxygen training.