(A prop IIRC) was well intentioned but incredibly broad. Literally every single business in the state has that warning somewhere near the entrance.

I’m not a rocket surgeon or nothing fancy like that, but I think the potential for cancer in those components would be if they were finely powdered and inhaled over a period. Because those chemicals are mostly fancy names for things like “rust” and “sand.”

https://www.hunker.com/12212001/vermiculite-vs-perlite

My understanding:

The chemicals you listed are pretty innocuous.

The main problem with perlite is breathing in the dust, which is mostly silicon dioxide (essentially sand). The small particles can lodge in the lungs and cause silicosis. This is going to be rare among gardeners, I would think.

From: http://www.therm-o-rock.com/images/uploads/Therm-O-Rock-East-Perlite-SDS.pdf

I would think that once the perlite is mixed in the soil, the risk would be essentially non-existent. When removing from the bag, mixing, etc., I would recommend wearing a mask to keep from inhaling the dust and try to be careful to minimize the release of dust.

California is required to list all carcinogenic substances for consumer protection. At times the warnings may seem excessive. I would bet that most cancer cases related to perlite are due to mining the material. Nevertheless, taking precautions is good practice for everything we come into contact with.

Most perlite sold in the US is expanded perlite.

Take the perlite, make small bits of it, and expose to high temperature. The moisture and trapped gases make for that white lumpy stuff you find in potting mixes and in plants from greenhouses. It helps both drainage as well as to trap some moisture for plant roots.

It's nasty stuff. Does cause cancer and lung problems because of the small bits of silica that go airborne when you dump the stuff into a bin for mixing with other soil constituents.

The cancer-causing action is like that of asbestos: The actual chemical, as far as I know, is safe. But the small particulates do not dissolve, are not assimilated, but physically damage cells so that some become cancerous. (Unlike, say, tar in cigarettes, which contains chemicals that are teratogenic or trigger cell mutations through chemical action.)

I take the California law with a grain of salt. If it's not proven safe pretty much by everybody, it's a possible carcinogen. All it takes is one study to cast doubt on something's perfect safety.

I'm buying him some masks and gloves.

I have a 12 year old girl. Should I worry about transporting this with her in the car?
I have time off so I can make a trip to deliver this to him. Should I worry about residual effects of having it in my car?

you have an accident or some other cause of her getting buried under the perlite.

I'm being facetious - but only to make a point. The warning label is overkill (IMO). Don't breathe it. And especially don't breathe a LOT of it. If it's in a bag, it should be fine. Plenty of people use it all the time without masks or gloves and never suffer a bit.

I doubt that even a dedicated gardener is going to inhale that level of dust. Gardeners do other things besides handle perlite.

A quick search turned up these 2 items.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4002636/
Perlite toxicology and epidemiology – a review

and

https://oehha.ca.gov/media/downloads/proposition-65/p65111017_0.pdf
STATE OF CALIFORNIA
...
CHEMICALS KNOWN TO THE STATE TO CAUSE CANCER OR REPRODUCTIVE TOXICITY
November 10, 2017

(It doesn't list perlite)

I've only given these a cursory glance, take it with a mountain of salt.

don't just stick that warning on packages "just in case". Then if the State of California does put this or that ingredient on the list later on, they're covered.

And when we were kids they let us play in "sand boxes"!

I'm guessing that the lawyers are going to have all manufacturers put that warning on all products just to be safe legally.

In other words, the warning is meaningless.

The problem is the dust in the raw product which can be inhaled.

It shouldn't be harmful in small amounts.

It could be a hazard for those who manufacture it because of the dust produced.

Once it is wet, it no longer produces dust.

It is as harmful as the dust produced during the dust bowl.

It was so broad as to be meaningless - any chemical that was ever suspected of being linked to cancer requires a warning, which is why practically every retail establishment has that vague warning sign (ok, I shop a lot at hardware and garden stores).

I use perlite a lot in gardening, especially for potted plants as it provides better drainage than plain potting soil. As others have pointed out, it can be very dusty when a bag is first opened, so I only work with it outdoors. The components listed in the OP aren't particularly toxic to touch - the bulk of it, silicon dioxide, is the main component in sand and glass and is chemically rather inert. (Fun fact: I have an old jar of Safeway chili powder that lists silicon dioxide as its 3rd ingredient.) Perlite is essentially volcanic ash so treat it that way - you don't want to breathe in large or even moderate amounts of the stuff but it's pretty much what you'd get if you crush volcanic rocks.