Joshs frogs false bottom, NEHERP LDL, and other similar products are all growstone, aka recycled glass, and the manufacturer's website says it is not safe for animals. These products are also very dusty and require both respiratory and skin protection to handle. This dust contains powdered glass that cover the vivarium in microscopic glass particles. Why is a product clearly stated to not be safe for animals even being sold for this purpose? Rinsed hydroton or aquarium gravel are much safer.
The reason they have that warning is because of the silica content.
The same silica content that play sand has -- that is why play sand often has warnings on it that are the same! Yet we use play sand as a component in bioactive substrate and as substrate for hermit crabs, fish, and other animals. And people let their children play in it. That’s even what it’s sold for. (The warnings in California are even more dire!) All natural beach sand also contains silica and has the same risks.
Silicosis is very serious. It is a risk of any dusty material containing silica. Hydroton is an expanded clay product which also contains silica, and there is a risk in breathing the dust on it.
Glass and clay are essentially variations on the same minerals.
I work with ceramic material for a living, and we take many cautions in working with it. We never sweep, we only mop. We wear ventilators when handling dry clay. We never mix clay or glaze materials without using a huge filtering vent. We do not sand pieces except under the vent, wearing a mask.
You should be very careful breathing things like dusty excavator clay, decomposed granite, pumice, diatomaceous earth, and similar products also. It is also in almost all dirt (this is what caused all the respiratory issues during the Dust Bowl).
Silicosis is accumulative and once you breathe in silica the damage is done. You can have acute silicosis from breathing a lot at once, but there is also chronic, which is breathing smaller amounts over time.
You should rinse both expanded glass drainage and LECA or Hydroton, but one is not safer than the other. It is best to wear a mask when doing so to avoid breathing the dust.
Once the product is thoroughly washed, they are both safe. They are also used in a way that keeps them wet, and below a substrate layer, meaning breathing dust is not a risk to the pet.
Aquarium gravel is also safe, but very heavy. A silica free choice that’s lighter is a false bottom using egg crate and fiberglass mesh.
I hope this clears up any worries about the safety of expanded glass and expanded clay drainage layers!