According to the Environment Agency of England and Wales, around 55 million waste tyres are produced in the UK every year. On average, a tyre will be good for for around 20,000 miles before replacement is required. The EU-wide ban on the landfilling of tyres has compelled the development of various methods of processing waste tyres, but processing and disposing of the volume of waste tyres produced globally remains challenging and expensive. Many such processes are designed to extract any materials of potential value from waste tyres, such as steel, and reprocess waste rubber into a usable form for a range of applications where its properties are suitable.
Intermediate storage of vast quantities of tyres awaiting processing presents a number of issues in itself, not least the risk from arson, accidental ignition, or even spontaneous combustion. The Fforestfach tyre fire in Swansea occurred in 2011, with over 5,000 tonnes of shredded tyres stored in a factory burning for over three weeks. The fire created a large quantity of dark smoke, and nearby watercourses were polluted by the water used to fight the fire. Two company directors were each sentenced to six months in prison for storing waste without an environmental permit. The cause of the fire remains unknown.
Recent media attention has also highlighted problems with unscrupulous ‘waste companies’, typically offering cut-price tyre disposal. Such businesses typically disappear, leaving the owner of their rented facility with a building or yard stacked to capacity with waste tyres. Much more serious problems of this nature are known to exist outside of the UK, such as a tyre ‘graveyard’ in Kuwait, which contains over 7 million tyres, and is visible from space.
Why reclaim or recycle tyre rubber?
As we have seen, simply storing used tyres is ultimately unsustainable and a waste of a useful material, and burying them in landfills is illegal. Tyres can be reused for other purposes, either whole or after a shredding process. They can be used as an industrial fuel, typically mixed with other suitable fuels. Shredding of tyres allows for the recycling of the steel wire within them.
Construction and civil engineering applications for waste tyres
Whole tyres can be used in a number of applications, avoiding the expense of processing and minimizing the potential for environmental leaching. Waste tyres can be used to construct inexpensive culverts. They also make good retaining walls, and can be used to stabilise beaches and sloping ground. Earth-rammed tyres are sometimes used in retaining wall construction, in order to give greater strength. Tyre bales, in which tyres are first pressed in a baler and then wrapped with steel wire, are also used in construction. Their use is covered by the Environment Agency Regulatory Position Statement 085, which details the conditions under which they may be used without applying for an environmental permit. PAS108:2007 is the relevant standard for the production of tyre bales for construction usage. This standard was developed by WRAP (Waste and Resources Action Programme) in collaboration with the BSI (British Standards Institution).
Shredded tyres are also used in construction, typically for lightweight fill, land drainage applications and for recreational surfaces. The use of tyre rubber mixed with asphalt for road surfacing is a potentially important novel use. The mixture has been used on the A90 between Perth and Dundee.
Potentially harmful substances found in tyre rubber
A variety of heavy metals are found in tyre rubber. Zinc is the main metal of concern, being typically found in the form of zinc oxide at a level of around 1 to 2%. Although zinc is an essential trace element, large amounts can be toxic and can also have adverse environmental effects. Other heavy metals oxides, such as cadmium, lead and arsenic, are found at low levels, arising as impurities in the zinc oxide. Polycyclic aromatic hydrocarbons (PAHs) are also found in tyre rubber.
Concerns that degradation may release contaminants into the environment
The long-term degradation of tyres in water was studied in a 2003 paper by Fenner and Clarke (from Cambridge University and the Environment agency, respectively). They found that in a river or marine environment some leaching occurred, with cadmium being the most significant pollutant. The overall conclusion was that tyres could be used for aquatic structures if due regard was paid to local conditions.
The Environment Agency Regulatory Position Statement 085 indicates that tyre bales should not be used below the water table in Source Protection Zones 1 or 2, nor in very acidic environments (peat bogs, etc.) where the soil pH is 5 or less, in order to reduce the risk of leaching. No more than 1,000 bales can be stored at any one location, and they must be used within 3 months. These regulations are there to ensure that tyre mountains are not produced. Written fire control measures are required.
In general, intact tyres less prone to leaching than shredded tyre rubber. A 2003 paper by Canadian researchers (Birkholz et al.) showed that fresh shredded tyre produced a moderate toxic threat to aquatic species, if run-off was not diluted. However, the danger receded as the material aged.
Use of waste tyre rubber in recreational surfaces
Shredded tyre rubber has been used in a number of countries, including the UK, to provide a shock-absorbing surface for children’s play areas and synthetic turf playing fields. The surface reduces the risk of injuries compared to concrete or asphalt. Nowadays. shredded tyres tend to be preferred to sand in play areas. In recent years concerns have been expressed about the potential toxicity of shredded tyres. Various forms of tyres can be used:
• Loose fine tyre shreds (‘crumb’) that produce a surface that can be raked.
• Shreds that are combined with a binder and poured to give a permanent surface.
• Factory-produced tiles that are glued to the playground surface.
A number of studies have looked at the potential risks from shredded tyres in children’s playgrounds. There is a danger of young children ingesting small amounts of playground material. Young children are typically more at risk from a given amount of toxic material than adults, due to their smaller size and the fact that their bodies are still developing.
A detailed study was carried out by the Californian OEHHA (Office of Environmental Health Hazard Assessment) in 2007. The study considered whether children would be at risk from either eating tyre material or else touching the material and then touching their mouth. A chemical solution that mimicked a child’s stomach content was used. Wipes were used to mimic touching tyre material.
It was concluded that a single episode of eating 10g of rubber material would be unlikely to cause serious harm. More concerning was chronic exposure from continuously touching tyre material. The wipe studies indicated five compounds present on the wipes at levels significantly greater than the background level: chrysene, fluoranthene, phenanthrene, pyrene and zinc. The first four compounds are all polycyclic aromatic hydrocarbons (PAHs), of which chrysene is classified as carcinogenic. The chrysene present was estimated to give an increased cancer risk of 2.9 in one million for a child using a playground from age 1 to 12. In the USA, an increased risk of cancer of greater than 1 in one million is considered the point at which action should be taken. The authors admitted that there are many uncertainties in their figure for increased cancer risk. Adverse effects due to zinc were considered unlikely.
Guinea pig skin sensitisation tests were also carried out on the material, but showed no evidence of such problems. It was concluded that sensitisation was unlikely to be a concern.
An earlier (2003) study by Canadian researchers (Birkholz et al.) suggested that the danger of cancer from tyre shreds was minimal. The researchers extracted the tyre with solvent, and then tested the resulting mixture for genotoxicity. The results were negative. No attempt was made to identify individual components in the extract mixture.
The non-profit US organisation EHHI (Environment and Human Health, Inc.) produced a long report in 2007 on the use of shredded tyres in artificial turf, playgrounds, etc. They contracted analytical work to the Connecticut Agricultural Research Station, who looked at traces of volatile compounds by gas chromatography / mass spectroscopy techniques. The majority of the volatile compounds were not identified, but four major components were definitely identified by confirmatory tests: benzothiazole, butylated hydroxyanisole (an antioxidant), n-hexadecane and 4-(t-octyl)-phenol. Of these, butylated hydroxyanisole is classed as a carcinogen in some US states, despite the evidence being far from convincing. It is a widely-used food additive in Europe, so exposure from food is probably more significant than that from tyres. Leaching studies showed, as expected, that zinc was the main metal leached. It is strange that the EHHI report did not identify the PAHs normally seen in other studies, such that by the OEHHA, which must raise questions about the analytical methodology used.
The EHHI report considers other studies, including the OEHHA study, but considers them inadequate, possibly understating the risks. The authors recommended a moratorium on shredded tyres in playgrounds and sports fields. They also recommend that they should not be used on very hot days, when the risk from volatile chemicals is greatest.
The use of tyre rubber ‘crumb’ in artificial turf on football pitches has been linked to cases of cancer in US women footballers. The cancer cases are claimed to particularly affect goalkeepers, who would be expected to have the greatest exposure, due to their diving onto the ground. No scientific studies have yet been published to confirm whether there is indeed an excess of cancer among footballers, and whether it is linked to artificial pitches containing tyre ‘crumb’. A 2011 analytical study by the Connecticut Department of Public Health (Gingsberg et al.) did not find any substances derived from tyre ‘crumb’ at levels high enough to be considered dangerous. However, the study was only based on five ‘crumb’ rubber playing fields, so was limited in scope.
The EPA has recently launched a thorough investigation into the safety of shredded tyres in recreational surfaces, in order to establish some definitive conclusions. Currently, the EPA is not giving any recommendations as to whether or not tyres should be used. However, the only sure way to prove the safety / danger would be a long-term study comparing the health of large groups of children who regularly played on shredded tyre surfaces to other large groups who were not so exposed. Similar studies would be needed for adults playing on synthetic football pitches. In the meantime, it seems sensible for parents to ensure that children wash their hands after coming in from playing.
Tyre rubber in equestrian arenas and sand maneges
Tyre rubber crumb is sometimes added to the sand in equestrian arenas, the use being permitted under current Environment Agency waste regulations (Waste Exemption: U8 use of waste for a specified purpose). The additive makes the sand more responsive (‘bouncier’) under a horse’s hooves, and also prevents sand ‘caking’.
There may be similar risks from rubber in horse arenas to that used in playing fields. Falling riders may get particles in their mouths. However, the particle size here is generally greater than the ‘crumb’ rubber used in artificial turf. Further research is needed to establish what the real risk (if any) is to the horses or riders.
Conclusions on waste tyre use
The re-use of tyre and tyre rubber is part of moving towards a sustainable society. However, health concerns need to be addressed in order that the use of tyre waste is acceptable. While the majority of waste tyre use seems to be low risk, it may turn out that restrictions are needed on the use of tyres in children’s playing areas and on sports fields. Research to date suggests that there may be a small, but not insignificant, problem. Further research is needed so that more definite answers can be given.
The controversies over tyre ‘crumb’ are an illustration of a widespread conflict between the need to recycle and fears of contamination leading to risks to human health. Such conflicts are not confined merely to tyres, but affect many other areas of modern life and industry.
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