Carcinogenic Car Tires?

Is there evidence to suggest that growing vegetables in old car tires filled with soil will contaminate them with carcinogenic substances?

Yes—but the evidence supports a plausible, probably low but uncertain risk, not a demonstrated finding that vegetables grown in intact old tires are necessarily contaminated with carcinogens.

What tires can contain

Tires contain complex mixtures of rubber, carbon black, oils, antioxidants, vulcanizing chemicals, metals, and other additives. Chemicals of concern include:

• Polycyclic aromatic hydrocarbons (PAHs), some of which are carcinogenic.
• Zinc and smaller amounts of metals such as lead, cadmium, chromium, and copper.
• Rubber additives such as benzothiazoles, 6PPD-related compounds, and vulcanization accelerators.

The EPA has found PAHs and metals in tire-derived rubber, although its exposure studies—primarily involving tire crumb used in athletic fields—found that only small fractions of many metals were bioaccessible under simulated exposure conditions. Those findings do not directly establish the safety of tire planters for food crops.^[1][2][3]

What is known about plant uptake

There is direct experimental evidence that some tire-derived chemicals can enter edible plants. In a hydroponic lettuce experiment, compounds associated with tire wear—including benzothiazole, diphenylguanidine, HMMM, 6PPD, and 6PPD-quinone—were taken up and transported into leaves. The study used controlled solutions containing tire-derived compounds rather than whole tires in ordinary garden soil, so it demonstrates biological possibility rather than proving that a typical tire garden produces harmful food.^[4][5]

A major limitation is that there appear to be few—or no well-controlled field studies measuring carcinogens in vegetables grown in intact tire planters over multiple seasons. McGill’s review reached the same basic conclusion: meaningful leaching is plausible, but the amount entering soil and vegetables from an intact tire is not well established. PAHs tend to be poorly water-soluble, whereas some metals and other additives may be more mobile.^[6]

Why intact tires differ from tire mulch

An intact sidewall or tread has relatively little surface area in contact with soil, and many chemicals remain bound in the rubber. That likely makes it less concerning than:

• Shredded or crumb rubber mixed into soil.
• Tires that are cut, abraded, or crumbling.
• Tire rubber exposed to acidic conditions.
• Burning tires or using tire ash.
• Long-term use in hot, sunny, wet conditions.

However, “less concerning” is not the same as “free of risk.” Aging, weathering, repeated wetting, and physical degradation can increase release of metals and organic additives. The strongest recent plant-uptake evidence concerns tire particles and leachates, not intact tires, so applying it quantitatively to a tire planter would be uncertain.^[7][8][4]

Practical recommendation

For vegetables intended for regular consumption, I would avoid using old tires as the actual soil container, especially for:

• Root crops such as carrots, potatoes, beets, and radishes.
• Leafy greens such as lettuce, spinach, and kale.
• Long-lived crops or perennial herbs.
• Tires that are cracked, deteriorating, oily, or heavily weathered.

The safer approach is to use untreated wood, masonry, galvanized metal designed for garden use, food-grade containers, or grow bags. If you want the tire appearance, put a separate food-safe growing container inside the tire so the soil does not contact the rubber.

If tires are used despite the uncertainty, reduce exposure by using intact tires only, keeping edible plant material away from the tire wall, avoiding root crops and leafy greens, using clean tested soil, and washing produce thoroughly. A plastic liner may reduce direct contact, but it introduces its own material and aging questions rather than eliminating uncertainty.

Bottom line

There is evidence that tire-derived chemicals—including some potentially carcinogenic PAHs and other toxic compounds—can leach under some conditions and that certain tire-derived chemicals can be taken up by lettuce. There is not yet strong evidence showing that ordinary vegetables grown in intact old tires routinely contain carcinogenic substances at dangerous concentrations. Given that safer containers are readily available, tires are best reserved for ornamental plants—or used only as an outer decorative shell around a separate food-safe planter.^[1][4][6]

⁂

• https://www.epa.gov/chemical-research/public-webinar-part-1-tire-crumb-rubber-characterization  
• https://www.epa.gov/chemical-research/federal-research-recycled-tire-crumb-used-playing-fields-and-playgrounds 
• https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100QM90.TXT 
• https://pmc.ncbi.nlm.nih.gov/articles/PMC9835885/   
• https://pubs.acs.org/doi/10.1021/acs.est.2c05660 
• https://www.mcgill.ca/oss/article/health/are-vegetables-grown-tire-gardens-safe-eat  
• https://pubs.acs.org/doi/pdf/10.1021/acs.est.2c05660 
• https://downloads.regulations.gov/EPA-HQ-OPPT-2024-0403-0056/content.pdf 
• https://pmc.ncbi.nlm.nih.gov/articles/PMC10221668/ 
• https://louisville.edu/cepm/pdf-files/newpg28 
• https://ask.extension.org/kb/faq.php?id=784173 
• https://ask.extension.org/kb/faq.php?id=305496 
• https://www.mdpi.com/2071-1050/17/12/5433 
• https://swyg.nmsu.edu/2015/050215.html 
• https://www.echocommunity.org/en/resources/b1d9d1eb-afcb-4729-ae31-950e13872d1d 
• https://gardeningbeyond.com/how-to-make-a-tire-garden/ 
• https://www.eurekalert.org/news-releases/1111353 
• https://www.echocommunity.org/en/resources/39a9a5a2-47a2-4870-9beb-350ff6e7fbfa 
• https://gardeningbeyond.com/how-to-make-tire-garden/ 
• https://19january2021snapshot.epa.gov/sites/static/files/2019-12/documents/setac2019_tire_crumb_poster_final.pdf 
• https://www.epa.gov/chemical-research/tire-crumb-questions-and-answers 
• https://19january2021snapshot.epa.gov/chemical-research/tire-crumb-questions-and-answers_.html 
• https://www.epa.gov/sites/default/files/2019-08/documents/tc_public_webinar_-_august_6_2019.pdf 
• https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100X5RO.TXT 
• https://www.epa.gov/system/files/documents/2024-04/epa-atsdr-tire-crumb-study-response-to-external-peer-review-comments.pdf 
• https://19january2021snapshot.epa.gov/sites/static/files/2019-08/documents/synthetic_turf_field_recycled_tire_crumb_rubber_research_under_the_federal_research_action_plan_final_report_part_1_volume_1.pdf 
• https://www.uml.edu/docs/Tire-crumb-2017_tcm18-385461.pdf 
• http://i2.cdn.turner.com/cnn/2017/images/01/25/epa.on.chemicals.in.turf.also.found.in.soil.pdf 
• https://downloads.regulations.gov/EPA-HQ-OPPT-2024-0403-0110/attachment_8.pdf 

Goji Berry Question

Editor:

Picking Goji berries on bushes right next to a major local traffic route, I have noticed an abundance of berries from bushes right next to the road, in comparison to bushes set back from the same road on the same property under the care of the same gardener. Could increased carbon dioxide in the air have something to do with it?

Yes, but probably not mainly because of carbon dioxide. Roadside plants can sometimes benefit from warmer microclimates, extra nitrogen deposition, altered water availability, and reduced competition—while traffic pollution can also harm them. The unusually heavy fruiting is more likely a combination of these factors than a simple “more CO₂ makes more berries” effect.

Carbon dioxide: possible, but weak explanation

Vehicle exhaust can create short-lived CO₂ increases very close to a busy road, especially during calm conditions. Because goji is a C3 plant, elevated CO₂ can initially increase photosynthesis and biomass production. Controlled studies on goji found that CO₂ enrichment affected photosynthesis and fruit development, and could increase plant or fruit biomass under some conditions. However, the experimental enrichment was around 700 ppm for long periods, whereas outdoor roadside CO₂ increases are usually intermittent and rapidly dispersed. Long-term elevated CO₂ also caused reductions in goji fruit sugars and some secondary metabolites.^[1][2][3]

So, CO₂ could contribute locally, but it would be difficult for ordinary traffic-related CO₂ alone to explain a dramatic difference in berry abundance.

More plausible roadside effects

Several other differences may be important:

• Nitrogen deposition: Vehicle exhaust contains nitrogen oxides, and roadsides can receive additional nitrogen through dry deposition. Nitrogen can stimulate leafy growth and, if the plant is not excessively stressed, potentially support more flowering and fruit production. Road-related effects on plant nitrogen and growth are commonly strongest within roughly 50–100 metres, though they can extend farther.^[4][5]
• Heat and earlier development: Pavement and traffic create a warmer microclimate. In Edmonton, a roadside shrub may warm earlier in spring and remain warmer into autumn, potentially extending the growing season and improving fruit ripening.
• Light exposure: The roadside bushes may simply receive more direct sunlight or less shading from other plants, fences, buildings, or trees.
• Water conditions: Roadside grading, compacted soil, snowmelt, runoff, irrigation patterns, and reduced competition from surrounding vegetation can all alter water supply. Goji can fruit heavily when it receives adequate moisture without excessive shading.
• Pruning and browsing: Even subtle differences in pruning, winter damage, rabbit browsing, or shoot age can strongly affect goji fruiting. Goji often fruits on current-season growth, so a plant producing more vigorous new shoots may carry many more berries.
• Pollination and flowering conditions: Wind exposure, insect activity, and the timing of flowering may differ between the exposed roadside and the more sheltered part of the property.

Pollution may also be misleading

Traffic exposure is not uniformly beneficial. Ozone, nitrogen oxides, fine particles, road dust, salt, and hydrocarbons can damage leaves, alter stomatal function, reduce photosynthesis, or affect plant water relations. Roadside vegetation studies report both stimulated growth in some species and physiological damage in others.^[5][6][4]

A roadside plant can therefore look vigorous and fruit heavily while still accumulating contaminants on its leaves and fruit. I would wash the berries thoroughly and avoid consuming berries from a location with visible dust, heavy winter-salt accumulation, or direct splash from the roadway. For berries intended for regular consumption, the set-back bushes are the safer choice unless the roadside plants have been tested.

How to test the explanation

You could distinguish CO₂ from the other factors with a simple comparison:

• Select several roadside and set-back bushes of similar age and size.
• Record direct sunlight, soil moisture, irrigation, pruning history, shoot length, flowers, and berries per metre of branch.
• Measure berry count and total berry mass separately; abundant small berries may indicate a different response than fewer large berries.
• Compare leaves for dust, salt injury, chlorosis, and pest damage.
• Take soil samples from both zones and compare nitrate, electrical conductivity, pH, organic matter, and available phosphorus.
• If possible, place a few comparable potted goji plants in roadside and set-back positions while giving them identical soil and water.

If the roadside potted plants also outperform the sheltered ones, the microclimate or air exposure is probably involved. If the effect disappears when soil, water, and sunlight are equalized, the cause is more likely soil fertility, drainage, heat, or competition than CO₂.

Bottom line: traffic-related CO₂ may play a minor supporting role, but increased nitrogen deposition, warmth, sunlight, water conditions, and reduced competition are more plausible explanations for the heavier goji crop.

⁂

• https://pmc.ncbi.nlm.nih.gov/articles/PMC7991576/ 
• https://academic.oup.com/treephys/article/39/6/1032/5368476?searchresult=1 
• https://www.mdpi.com/2223-7747/10/2/321 
• https://publications.naturalengland.org.uk/file/5064684469223424  
• https://publications.naturalengland.org.uk/file/135001  
• https://www.nature.com/articles/s41612-020-0115-3 
• https://www.tandfonline.com/doi/full/10.1080/02757540.2020.1791102 
• https://nora.nerc.ac.uk/id/eprint/5621/1/HonourN005621PP.pdf 
• https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.802936/full 
• https://pdfs.semanticscholar.org/a5c7/0a639e344e9ccf181bad68d7f7cc7858022d.pdf 
• https://files.sdiarticle5.com/wp-content/uploads/2025/06/Revised-ms_AJEE_138150_v1.pdf 
• https://www.witpress.com/Secure/elibrary/papers/UT03/UT03032FU.pdf 
• https://pmc.ncbi.nlm.nih.gov/articles/PMC6060415/ 
• https://www.fs.usda.gov/psw/publications/mcpherson/psw_2013_mcpherson006_baldauf.pdf 
• https://eprints.soton.ac.uk/382651/1/StreetscapingManuscript_EnvPolFullPaper_Submitted.pdf 

An Angle on Evolution

Editor

I have a hunch that life, as we know it in the carbon world, comes into being more or less spontaneously, when the chemical, physical, etc. conditions allows for interactions that can maintain new entities and their interactions. As happened when the Earth's atmosphere became oxygenated some 2 billion years ago.

Your hunch lines up well with how many origin-of-life researchers now think about it: life doesn’t require a miracle spark so much as the right regime of chemical and physical conditions that allow self-maintaining, interacting networks to arise and persist.^[1][2]

Spontaneous emergence under the right conditions

Modern “systems chemistry” and abiogenesis work increasingly treats life as a dynamical state that can appear when:

• There’s a supply of energy and matter (e.g., redox gradients, sunlight, geothermal heat).
• There’s a rich enough inventory of elements and molecules (especially C, H, N, O, P, S).
• There are environments that concentrate, cycle, and protect those molecules (pores in rocks, tidal zones, hydrothermal vents, ice interfaces, etc.).
• Reaction networks can form that are self-propagating and self-correcting (autocatalytic cycles, compartmentalized chemistries, simple replication).^[2][1]

In that view, once conditions cross certain thresholds, the spontaneous appearance of simple, fragile replicating or self-maintaining chemical systems becomes plausible, even expected. Those systems then “complexify” via selection-like dynamics into more robust, information-rich life.^[1]

This matches your intuition: life emerges when the environment allows interactions that can maintain new entities and their interactions over time.

The Great Oxidation Event as a regime shift

Your oxygen example is a great illustration of a planetary-scale regime change that opened new “life possibilities”:

• Before ~2.4–2.3 billion years ago, Earth’s atmosphere and shallow oceans were essentially oxygen-free (anoxic).^[3][4][5][6]
• Cyanobacteria had already evolved oxygenic photosynthesis, but for hundreds of millions of years the O₂ they produced was consumed by reduced minerals and gases (iron, sulfide, methane, etc.).^[5][7][8]
• Around 2.46–2.3 billion years ago, oxygen production finally outstripped these chemical sinks, and free O₂ began to accumulate permanently in the atmosphere and surface oceans—the Great Oxidation Event (GOE).^[4][6][9][3]
• This transformed Earth from a weakly reducing to an oxidizing world, eventually reaching perhaps up to ~10% of modern oxygen levels by the end of the GOE.^[6][10]

That shift didn’t “create” life—life was already there—but it reconfigured the chemical landscape:

• It was toxic to many anaerobes and likely caused a major extinction.^[8][11]
• It enabled aerobic respiration, a far more energy-efficient metabolism, which later supported larger, more complex cells and multicellular life.^[12][3]
• It changed global geochemical cycles (iron, sulfur, carbon, nitrogen), creating new niches and new kinds of possible biochemistry.^[7][5]

So the GOE is a concrete example of your idea: once atmospheric and oceanic chemistry crossed a threshold, a whole new class of self-maintaining, oxygen-using biological systems could arise and dominate.^[3][12]

Is life “inevitable” under the right conditions?

Many researchers now phrase the question as: Given Earth-like conditions over geological timescales, is some form of self-maintaining chemistry (life) a likely outcome?

Current thinking suggests:

• The basic physical requirements are fairly generic: liquid water, moderate temperatures, and access to CHNOPS elements.^[2]
• Under those conditions, a wide range of prebiotic chemistries can produce building blocks (amino acids, nucleobases, lipids) and simple reaction networks.^[13][2]
• The step from “interesting chemistry” to “self-propagating, evolving system” is still not fully solved, but the trend is toward seeing it as a natural dynamical transition in complex chemical systems, not a one-off accident.^[1][2]

That doesn’t prove inevitability, but it does support your intuition: when the environment allows certain kinds of interactions to persist and amplify, life-like organization can emerge “more or less spontaneously” in the sense of being a natural consequence of those conditions.

If you’d like, we can drill into specific mechanisms people propose for that transition (e.g., autocatalytic sets, metabolism-first vs. replication-first scenarios, or how hydrothermal systems could act as “reactors” for early life).

⁂

• https://pmc.ncbi.nlm.nih.gov/articles/PMC3718341/    
• https://pmc.ncbi.nlm.nih.gov/articles/PMC12489502/     
• https://academic.oup.com/bioscience/article/66/3/189/2468670    
• https://academic.oup.com/bioscience/article-abstract/66/3/189/2468670?redirectedFrom=fulltext  
• https://pmc.ncbi.nlm.nih.gov/articles/PMC8403206/   
• https://en.wikipedia.org/wiki/Great_Oxidation_Event   
• https://www.nature.com/articles/s41561-025-01708-1  
• https://www.imperial.ac.uk/news/171487/imperial-scientist-explains-oxygen-triggered-earths/  
• https://www.science.org/content/article/what-triggered-one-most-dramatic-changes-earth-s-history 
• https://eos.org/science-updates/how-great-was-the-great-oxidation-event 
• https://pmc.ncbi.nlm.nih.gov/articles/PMC8960603/ 
• https://asm.org/articles/2022/february/the-great-oxidation-event-how-cyanobacteria-change  
• https://www.britannica.com/science/life/The-origin-of-life 
• https://news.mit.edu/2026/some-early-life-forms-may-have-breathed-oxygen-before-filling-atmosphere-0206 
• https://artsandsciences.syracuse.edu/earth-sciences-department/ees-news/rock-record-illuminates-oxygen-history/