Over the years, people have raised the concern with me that growing vegetables in old tires can lead to heavy metal contamination of the plants, and therefore would represent a health risk to the very people we are trying to help. I have struggled to get clear information about the risks, which has included an ongoing dialogue with several people related to Educational Concerns for Hunger Organization [ECHO].
In January 2016, ECHO published an article in their ECHO Development Notes titled "Tire Contaminants from a Container Gardening Perspective" which reviewed a number of articles, particularly one from the United Kingdom and one from Nepal. The one from the UK, "Environmental Health Implications of Heavy Metal Pollution from Car Tires" by J.M. Horner (1996) cannot be downloaded online but the abstract can be found at [Environmental Health Implications of Heavy Metal Pollution from Car Tires"] and a request can be made for the full article.
The article from Nepal, "Studies and Determination of Heavy Metals in Waste Tyres and Their Impacts on the Environment" by P.R. Shakya et al (2006) is downloadable from the following link [Nepal Tire Contaminants Study]
You can download the ECHO article directly from a Google search using the title, "Tire Contaminants form a Container Gardening Perspective." Look for the [PDF] initials right before the title. Other avenues for the article require you to be a member of the ECHO Community (which is free and an excellent thing to be).
I have a whole lot of other downloaded articles related to contamination, some of which I have worked through somewhat thoroughly, others that I just read the abstract. There is a study from the 1990's commissioned by the Ontario government that tested the effects of tires soaking in fresh water. The researchers generally found very little toxicity to trout fingerlings [Ontario study of Aquatic Environments]. I found another study today that also had a focus on the aquatic effect of tires, but with a more negative result, "Rubber Tire Leachates in the Aquatic Environment" by Joyce. J. Evans in Reviews of Environmental Contamination and Toxicology. The following site will sell you the chapter for $29 and the whole book for $99. You can look at the first couple of pages for free: [Rubber Tire Leachates in the Aquatic Environment]. The difference between the two seems to be that the Ontario study looked at the effect of whole tires on aquatic environments in which the whole tires are submersed. The second study looks at what happens when the rubber is ground up and water leaches through the concentrated, ground up tire and then flows into the local aquatic system.
The two most recent studies that I have looked at a little more thoroughly both come from Sweden. "Metal Emissions from Brake Linings and Tires: Case Studies of Stockholm, Sweden 1995/1998 and 2005" by David S.T. Hjortenkrans (2007). You can download it at [Swedish Article]. The title looks complicated but the basic information about concentrations is simple and clear and found on page 5227, 2nd paragraph under the tables. I just found the 2nd Swedish article today. The title is great, "When the Rubber Meets the Road: Ecotoxicological Hazard and Risk Assessment of Tire Wear Particles" by Anna Wik (2008) [When the Rubber Meets the Road].
If I told you I had read all of these articles, even just the ones I have downloaded and printed out, from front to back, I hope you would not believe me. If I told you that I have understood at least three-quarters of what I have read, you might believe me, but I would still be lying. I guess I've gotten maybe half of it all at best. This is NOT my area of expertise. Chemistry, and especially Biological Chemistry is fundamental to understanding soils and growing plants. I think I am solid enough when it comes to understanding pH, Cation Exchange Capacity and at least the basics of the whole Nitrogen:Carbon relationship. But tires are complex. They are not just natural rubber. In fact, the excerpt from the Rubber Tire Leachates study includes the observation that tires today are mostly made of synthetic substances than from natural rubber. The Swedish Doctoral Thesis ("When the Rubber Meets the Road"), lists the following materials for the tire tread: synthetic and natural rubbers (40-60%), Carbon black and silica (20-35%), Mineral oils (15-20%), Sulphur (1%), Zinc oxide (1.5%). There is something like 2% of three or four other types of substances as well.
Here's what I think I do understand.
None of these articles are actually looking at the way we use tires.
In the ECHO arcticle, Ben Fisher and other ECHO staff members had to struggle with the same thing. They had to extrapolate from the information provided, applying it to the vegetable tire scenario as best they could, without knowing exactly how everything fits within a very different context.
The article from the UK used fragments of tire leached with solutions with a pH of 2.5. That study was simulating the conditions of a pile of waste tires exposed to acid rain.
The Nepal study actually burned the tire samples and measured the heavy metals in the ash. That simulated very well what happens in Nepal where many families use cut up tires as a fuel source for warmth. Not much to do with how we use them.
"When the Rubber Meets the Road" looked at the bits that come off the tires and contaminate the environment. Like the Nepal study, they measured the total concentration of heavy metals in the tire treads. I like the fact that they look at the treads, but we are not rolling our tires on the ground to grow vegetables.
The Ontario study was similar to using tires for vegetables in the sense that they looked at the impact on the whole, un-fragmented tire. They also examined the effects of contaminants leaching from the tire as water flowed over the material using living beings. They used trout fingerlings, rather than pepper plants, but still, there is something about this study that feels like it matches better than the others.
RESULTS OF MY READINGS
I understand that there are four heavy metals that are of concern which have been found in tires. They are in order of scariness to me, lead, cadmium, chromium and zinc.
Note: the following examples are given in grams. A gram is very small unit of mass, so obviously fractions of grams are even less. There are about 450 grams in one pound. There are about 28 g in one ounce. Medicines are almost always measure in milligrams, which is 1/1000th of a gram. PPM stands for "parts per million." What that means is that 1 ppm is 1/1,000,000 (one one-millionth) of a gram of heavy metal for each gram of tire material.
In all of the studies, the lead that was found in the tires did not come from the manufacturing. It accumulates on the tires as the roll along the road, picking it up from other sources. Brake linings, for example, have a significant amount of lead.
The UK study found concentrations of between 8.1 and 22.33 ppm (parts per million) lead in the liquid leached from the tire fragments.
On average, a truck tire weighs about 40 pounds, or about 18 kg. using the higher concentration, this gives a total of 0.40 g potential lead that would be leached out of a whole tire.
The Swedish study actually found less lead in their tires. They got a concentration of 1.1 ppm, the equivalent of a total of 0.0198 g in the whole pickup tire. Again, the Swedish study focused on the tire tread because they wanted to know what was coming off the tire and dispersing as a contaminant into the environment. For the sake of the argument, I am assuming that the concentration of heavy metals remains constant for the whole tire.
The UK study found up to 3 ppm of cadmium, the equivalent of 0.054 g per large pickup-sized tire. The Swedish study found 1.1 ppm of cadmium, the equivalent of 0.040 gram in the whole tire.
The UK study did not look at chromium. In the Swedish study, they found 8.6 ppm in the tread, which extrapolates to 0.155 g per tire, if concentrations remain constant for the whole tire.
(Ben Fisher in the ECHO article notes that at Redeemer University College they tested a tire sample and found 0.9 ppm cadmium, about the same as the Swedish study).
Zinc is a nutrient for both plants and animals, but in excessive concentrations, it can cause damage to both. If the soil is highly contaminated with zinc, it will probably kill the plants. On the other hand, if humans cease to be exposed to or to consume excessive Zinc, the symptoms will generally disappear, according to the information presented by Ben Fisher in the ECHO article.
In the UK study, they found as much as 6012 ppm, the equivalent of 108.2 g in an entire truck tire. The Swedish study found as much as 12,000 ppm, the equivalent of 206 g in the whole tire.
When we create vegetable tire gardens in the Yard Garden program, or in my home yard, for example, we are not shredding the tires, we are not completely submerging them in water and we are not exposing them to highly acidic conditions. We do cut off one sidewall, but the soil is not exposed in any way to that area that we slice. We also turn the tires inside out, so the soil is exposed to the tire tread. In this sense, the numbers from the Swedish study are the most applicable. On the other hand, once the tires are planted, the rubber in our tires is no longer "hitting the road" so we are not contaminating our soil with bits of tire that rub off from friction.
One of the article notes that biological action will continue to degrade the tire. Our goal is to create soils that are extremely biologically active, so this type of degradation will continue to occur inside along the tire tread, but it will occur without the aid of constant saturation with water, abrasion by wind, exposure to ultraviolet rays, or mechanical friction. The outside of the tire will continue to be exposed to sun and wind and that will result in certain levels of degradation but should not effect the soil inside the tire.
So, given that these articles suggest that the TOTAL amount of lead, cadmium and chromium is less than one gram in the whole tire each, AND that those fractions of grams are not generally exposed to being released from the rubber, it seems abundantly clear that these heavy metals do not pose any serious health risk. Zinc, although present in much higher amounts, is also highly unlikely to pose any real risk because 1) the zinc is ALSO bound up into the tread and if it leaches or degrades out, it is likely that it will do so at relatively slow rates; 2) the zinc may kill the plants rather than render them poisonous for us and 3) increased intake of zinc by families, up to a point, could potentially have positive health effects.
In terms of the cocktail of other chemicals that make up a tire, they may be of more concern. A brief e-mail conversation that I had with the Dr. Edward Berkalaar at Redeemer University College suggested that some of the hydrocarbons can be taken up by plant roots. On the other hand, a note in the ECHO article cited information demonstrating that potentially toxic hydrocarbons would be a high risk for contamination only at a high pH. Ben Fisher does not mention the cut-off pH that the article thought the hydrocarbons would become more active, but "high" in terms of pH would normally start at 8.0 and above.
The Ontario stud, that I liked the best of all of them, found essentially that old tires soaked in water did not effect trout fingerlings in any measurable way; however they did find that new tires soaked in water could cause significant mortality. They got to where they could pinpoint that the substance associated with the mortality was one of the hydrocarbons--one or more of the "mineral oils" mentioned above.
Tires on the whole pose serious environmental risks. Disposal of them as trash where hundreds and even thousands are dumped in one relatively small area clearly has the potential to cause a serious environmental disaster. Grinding the tires up and then exposing them to sun and rain understandably creates conditions for significant heavy metal contamination. There are probably many additional ways to store or recycle tires badly.
Based on the information currently available, using a tire essentially intact for growing vegetables is not a risky way to make use of a very valuable resource.
The article from Echo Development Notes proposes some best practices and they are essentially correct. Here is my version of those best practices:
1) Choose tires that can be turned inside out so that the soil medium is exposed to the tread. This is the part of the tire that has been studied the most, so any additional information that comes out will probably be most applicable. To test a tire, lean lightly on the top of a tire standing straight up. If it bulges in easily, it should turn inside out easily.
2) Avoid tires that are retreads, or that have any kind of significant damage to the tread or the sidewalls. If possible, choose tires that are less than 10 years old. Tires in the States have the date they were manufactured printed on them...in very small print. Good luck with that part.
3) Wash the tire before using it to get any accumulated hydrocarbons and carbon black out of it and off the treads. A couple of good rains should do the job fine.
4) Use a soil mix high in organic matter. This is obviously best for the health of the plant, and the carbon will also bind readily with any offensive metals. If you have the means to measure the pH, make sure it hovers between 6.0 and 6.5. This is best for most vegetables and it seems to avoid the extremes that could promote either the leaching of heavy metals or the activation of the hydrocarbons. High organic matter will automatically tend to keep the pH within this range.
5) Include biochar in your soil mix. Again, we have seen that biochar really works in terms of vegetable production and it increases the binding of the offensive metals. Biochar also helps create a sponge texture in the soil mix which will keep your moisture supply more consistent.
6) Plant anything you want in the tires. Ben Fisher notes in the ECHO article that root crops are most likely to pick up heavy metals, fruits such as tomatoes or peppers are the least likely and leaf-producing crops are intermediate. But really, we're talking about minuscule amounts of metals spread out over a pretty big space. It really is not an issue, especially if you have followed the ideas mentioned above. In terms of the zinc, here is an abstract of a study that looked at the sensitivity of four vegetables to zinc concentration: [Zince Sensitivity]. Let me know in the comments if you find out more.
7) If you have the resources, paint the tires on the outside. It adds to the aesthetics and is a fun part of making a tire garden with young people. It will also help protect the outer part of the tire from degradation from the sun.
PLEASE SHARE ANY IDEAS OR OBSERVATIONS. I have been breaking my brain over this contamination issue for a long time. Conversations with Martin Price at ECHO and others kept me fully in the game of creating tire gardens, not to mention the beautiful production I've seen again and again in other yards and in our own. But I feel like I have finally got things really clear. I think I have read Ben Fisher's EDN article at least a dozen times, but finally today, with all of the other information I've digested, I really feel like I have a handle on it all.
Tires are by far the best option for container gardening. They are cheap, durable and can be transported long distances on the backs of pack animals.
Sack gardens are also an incredible idea, but that is another blog.
|Jenny, Keila, Annika and my tire garden in Barahona. We had green peppers, tomatoes, Okinawa spinach, basil, garlic chives and eggplants, not to mention the flowers. May 2015.|