Mad Scientist, where does the rubber go?

[Barry Wolk]

Where does it go as it wears off of our tires?

If the tread of an average 30" diameter tire is 6" wide and 1/2" thick and 94" long that's 282 cubic inches of rubber for one tire, correct? That's the equivalent of about a half-gallon of milk, by volume. 6 car tires equals about a cubic foot of the stuff. Just a guess, but a semi might contribute a cubic yard of the black stuff.

Where is it? Do you see wisps of it following other cars as you drive? How come there's no layer of rubber coating the front of our cars? Wouldn't you expect that a freeway that carries 166,000 cars and trucks a day would show some sign of black rubber accumulation?

Where is it all?

[Mad Scientist]

You know that is one of those things that I haven’t spent a lot of time researching. :)

But I suspect that if one were to carefully analyze samples of soil taken from alongside our highways traces of it would surely be found.

However I would evaluate this in the same way that as when Al Gore tells us about the millions and millions of tons of CO2 that we are putting into the air every day. This of course sounds horrible, until you realize that all these multi-tons of CO2 only amount to maybe .3% of the total atmosphere.

In the same way if you could somehow collect all this used rubber and put it into one pile you would have one really big pile. But spread that pile out more or less uniformly across the entire country and it would be hard to find any trace of it.

[Barry Wolk]

From a friend on another automotive site:

Its quite astonishing what you can find out the more you look in to this.

It appears that tires are a mix of materials, mostly synthetic and natural rubbers but also including carbon black, oil, sulfur, steel, and chemicals added as antioxidants, strengtheners, and fillers. They also contain varying amounts of potentially hazardous metals such as zinc, nickel, chromium, cadmium, and copper. As you drive, and especially as you corner and brake, your tires continually abrade against the road surface, and to some extent wear away just due to flexing as they roll along. This tire wear takes the form of rubber left on the road, heavy particles that quickly settle on the road and shoulder, and lighter particles that become airborne.

The rubber in car tires is typically about 40 percent natural--i.e., made from latex; there's even more latex rubber in truck tires. Putting a fine dust of latex into the air is a serious concern to those with latex allergies. Somewhere from 1 to 6 percent of the U.S. population has some sensitivity to latex, which can take the form of rashes, inflammation, asthma, and worse. (Health care workers who are constantly exposed to latex in the form of gloves and such have a higher rate of sensitivity, sometimes estimated at 15 percent or more.) While those values sound low and one can take steps to avoid latex exposure, if you live in an area with a lot of road traffic, airborne latex can make your life pretty miserable.

Those who aren't latex-sensitive don't get a break. Fine rubber particles, whether latex or synthetic, can lodge in your lungs and even enter your bloodstream. The Environmental Protection Agency has a whole category designated for such problematic particles: PM2.5, or particulate matter less than 2.5 microns in size. Excessive exposure can lead to reduced lung capacity, bronchitis, asthma, accelerated heart disease, and death. One study claims that nearly 60 percent of airborne tire particles are small enough to be easily inhaled. Radial tires produce a finer dust that's more hazardous than what's produced by older-style bias-ply tires; while that's plausible given tire construction differences, it's difficult to know for sure.

Tire dust that doesn't make it into the air can be problematic as well. Originally deposited on the pavement, it gets washed by rain into lakes and streams. Environmental scientist Alison Draper has shown that chemicals leaching out of tire dust can kill water organisms such as algae, plants, minnows, and snails. An Italian study found that the organic components of tire debris were toxic to frog embryos and to cultured human lung and liver cells.

How much rubber gets worn off of tires? Estimates vary widely--much depends on driving habits, vehicle weight, the type of road surface, and the type of tire. In the U.S., the amount is estimated to be on the order of 650,000 tons per year. A British study finds that about 10 to 20 percent of a tire's total weight is worn off during its lifetime, which works out to about 58,000 tons a year in the UK alone. Of that, the fraction consisting of those potentially toxic metals I mentioned is surprisingly high--36 tons of cadmium, more than 1,000 tons of copper, and nearly 3,300 tons of zinc. It is mentioned that pollution studies in the Los Angeles basin in the 1980s concluded that more than five tons of breathable tire dust were released into the atmosphere there each day, and there's no reason to think that figure's gone down since.

So just because tire dust isn't accumulating in huge black drifts along the roadside doesn't mean we can ignore it. What to do? One critical factor that determines how much rubber is lost from tires is inflation. Properly inflated tires create less rolling resistance than underinflated ones, build up less heat, flex less as they roll down the road, and release less latex into the environment. They also last longer and give you better gas mileage. So check your tire pressure next chance you get--not only will you be doing right by the environment, you might save a couple bucks.

[Barry Wolk]

Bump. Any other theories as to why we don't see what has to be there, somewhere?

[Doc]

You can add brake linings to the mystery as well--of course, that's a little easier to answer because most of us have seen where that goes: it's the source of the crud that's on the rims of our cars, especially on the front wheels on cars with disk brakes.

I think the reason we don't see the rubber dust--at least in the form of black plumes behind cars--is that the dust is so small it's not readily visible in the air (just like the dust in the air in our houses). Moreover, it comes off the tire at relatively high temperatures, so it floats up and doesn't settle directly onto the road surface. When we see it is when it settles in substantial amounts, but even then the particles are so small that we tend to call it "soot" and attribute it to our exhaust, not our tires. Anyone who has lived or worked near a major road, is familiar with the black dust that settles on their window sills, etc. That's largely tire dust, not exhaust particles. And, of course, it also settles on our cars--next time you wipe down the whitewalls on the Mark II, check the color of the "dust" on the tires--is it really the color of the soil in your area or is it darker/black (i.e. like the color of a tire)? On heavily traveled freeways with light concrete surfaces like we have in LA, the dark/black dust/soot that is clearly apparent along the medians, etc. is largely tire dust. Unfortunately, there is a growing literature that suggests tire dust is one of the major toxins associated with cities and urban life and is one of the reasons that kids who attend schools/live near freeways tend to have higher rates of asthma and related conditions.

[AXXel]

It's all in Perth Amboy...I know, I saw it there.