According to the Agency for Toxic Substances & Disease Registry , a division of the U.S. Department of Health and Human Services:
"Cadmium is an element. Its most abundant naturally-occurring isotope is non-radioactive. It is found in nature in mineral forms and is obtained for commercial uses principally from cadmium ore, called greenockite, which is commonly found in association with zinc ore. Commercial production of cadmium ore depends on the mining of zinc (ATSDR 1999)...
- Cadmium is resistant to corrosion.
- Cadmium metal and its oxides are insoluble in water...
- Cadmium, a heavy metal, is produced by refining zinc ores.
- Cadmium metal is practically insoluble in water but some cadmium salts are water soluble.
- Powdered cadmium will burn and can release corrosive fumes."
- Prevention is the key to managing cadmium exposure. No effective treatment for cadmium toxicity exists.
- For the general public, the primary source of exposure to cadmium is dietary. [Bolding mine.]
"Cadmium is found in many domestic products, e.g. tobacco products, phosphate fertilisers, polyvinyl chloride (PVC) products, photocells, petrol, oils, tyres, automobile radiators, some textile dyes and colours, electronic components, heating elements in electric kettles and hot water systems, batteries, and ceramic glazes." [Bolding is mine.]
According to this document, cadmium is used as a curing agent in tires.
U.S. OSHA (Occupational Health and Safety Administration) states that:
"Regardless of the route of absorption or the type of cadmium compound, approximately one half to one third of the body burden of cadmium is found in the kidneys after chronic low-level exposure, with the highest concentrations found in the renal cortex. (Ex. 8-086a, p. 168). After long term exposure, one sixth and one fifth of the body burden are found in the liver and muscles, respectively."
In studies on rats and mice, cadmium is also disruptive to the thyroid and adrenals glands. The effects of cadmium on the adrenal glands in rats were found to be more significant than the effects of lead.
According to a separate OSHA document:
"...most serious consequence of chronic cadmium poisoning is cancer (lung and prostate). The first observed chronic effect is generally kidney damage, manifested by excretion of excessive (low molecular weight) protein in the urine. Cadmium also is believed to cause pulmonary emphysema and bone disease (osteomalcia and osteoporosis). The latter has been observed in Japan ("itai-itai" disease) where residents were exposed to cadmium in rice crops irrigated with cadmium-contaminated water. Cadmium may also cause anemia, teeth discoloration (Cd forms CdS) and loss of smell (anosmia)."
The government usually deals in extremes, however, and we're talking about a few tires with potatoes planted in them in my back yard. What does all of this mean for me as someone who might want to use old tires to plant potatoes in (or make sandals from)? Are the concentrations that might be found in any foods I might grow in tires significant over time? After all, according to the folks at Backwoods Home Magazine, I could grow 25 pounds of potatoes in a stack of tires in one season. Is this a bad idea or not? And what about tires shredded and used as rubber mulch or playground surfaces? New issues arose as I looked more deeply.
"The Connecticut Agricultural Experiment Station report (See Appendix I), found out-gassing and leaching from synthetic turf rubber crumbs under aqueous ambient temperatures. Several compounds were present, but four compounds gave the highest responses on GC/Mass spectrographic analysis. The four compounds conclusively identified with confirmatory tests were: benzothiazole; butylated hydroxyanisole; n-hexadecane; and 4-(t-octyl) phenol. Approximately two dozen other chemicals were indicated at lower levels. These chemicals were released in laboratory conditions that closely approximate ambient conditions.
Those chemicals identified with confirmatory analytical studies at the Connecticut Agricultural Experiment Station study have the following reported actions:
- Benzothiazole: Skin and eye irritation, harmful if swallowed. There is no available data on cancer, mutagenic toxicity, teratogenic toxicity, or developmental toxicity.
- Butylated hydroxyanisole: Recognized carcinogen, suspected endocrine toxicant toxicant, gastrointestinal toxicant, immunotoxicant, neurotoxicant, skin and sense-organ toxicant. There is no available data on cancer, mutagenic toxicity, teratogenic toxicity, or developmental toxicity.
- n-hexadecane: severe irritant based on human and animal studies. There is no available data on cancer, mutagenic toxicity, teratogenic toxicity, or developmental toxicity.
- 4-(t-octyl) phenol: corrosive and destructive to mucous membranes. There is no available data on cancer, mutagenic toxicity, teratogenic toxicity, or developmental toxicity.
The study also detected metals that were leached from the tire crumbs. Zinc was the predominant metal, but selenium, lead and cadmium were also identified."The aforementioned addresses that nifty rubber surface that's become so popular in playgrounds and ballfields. This information might be worth taking to local park districts for consideration when playground renovations are being discussed.
During the course of investigation, I also happened upon some correspondence from Rufus Chaney of the Environmental Chemistry Lab at the USDA in which he discourages the use of tires for mulching and describes how in soil with low pH, zinc in old tires is leached more rapidly into the soil and may be responsible for killing vegetation. As for using tires as planters,
"Tires around soil as a raised bed garden has been used by many people. I have not heard of problems from that, but the surface area in contact with soil is small. In the short term, it may be little problem. But eventually the rubber degrades, Zn gets in the soil, and if the soil pH is 6 or below, uptake may be too much. Again, the higher the surface area, the more rapid the release of Zn and toxicity observation...
'Stable' is relative. Over time, microbes biodegrade rubber as a good source of energy. Toxicity to plants from ground rubber used as a mulch or a component or potting media, or burned tire residues in soils, have killed a wide range of plant species."
The editor-in-chief at Mother Earth News concurs:
"Short-term, yes, tire planters are OK, although the soil in black tire planters will probably get hotter than most plants would prefer. Long-term, no, because the tire rubber will slowly biodegrade and release zinc, carcinogenic PAHs (polycyclic aromatic hydrocarbons) and other toxic compounds into your soil.
Toxics from tires are an even greater concern in some recycled products, such as rubber mulch and artificial turf, that are made from shredded or pelletized old tires. Here is an excellent report about the hazards of recycled tires, by Sarah Lane at Progressive Kid.
Here's a helpful report from the Los Angeles County Extension about trace elements and contaminants in urban soi and reducing exposure to theml. Raised bed gardening is probably the best way to go for us urbanites, though after reading what I have, I am a lot less likely to use tires as planters. Personally, I'm planning to use the new and improved Square Foot Gardening model for our vegetable beds. For potatoes, I will probably just follow the recommendations given here.