Most of us could list at least half a dozen reasons why cars are bad for our health and bad for the environment. Cars are inefficient forms of transportation, they consume our rapidly declining stocks of fossil fuels, they are difficult to dismantle and recycle and they generate major pollutants such as hydrocarbons, nitrogen oxides, and carbon monoxide.

What might not be on that list, however, are tyres. Even if you are a car owner it is unlikely that you think about tyres at all until they need air, go flat or require replacing. Tyres are so fundamental to the function of every vehicle in the world – from the smallest Smart car to the biggest lorry and fastest jet – that it would never occur to many of us to ask what goes into them, or more importantly what comes out of them when the rubber meets the road. Yet pollution from tyre dust is linked to an expanding range of health problems including allergies, asthma and even heart disease.


Where does the tread go?

What goes into a tyre must also eventually come out. When a rubber tyre, bearing the weight of a vehicle, rolls across an asphalt or cement surface, tiny fragments of rubber, known variously as tyre dust or particulate matter, break off. Some become airborne and some are deposited at the side of the road, ready to be swept up by passing vehicles. Particulate matter is a very insidious form of air pollution and tyres contribute significantly to this form of pollution.

In the 1970s and early 1980s, when scientists working for the tyre industry and the Environmental Protection Agency (EPA) studied this phenomenon, it was concluded that tyre dust fragments were too large to enter the human lung and so presented no threat to human health. Today we know differently. Several studies published in the last decade have demonstrated that about 60 per cent of these fragments are so small that they can enter the very deepest parts of the human lung. The particles of greatest concern are those that measure 10 microns or less in diameter (a human hair, by comparison, is about 70 microns thick and the dust motes that can be seen spiralling through the air when the sun shines through the window measure around 10 microns).


A Dirty Mixture

The microscopic dust that comes off tyres contains a unique mixture of substances that have a more powerful effect on the body than naturally occurring dust. To form the rubber into hard-wearing vehicle tyres, an extensive range of chemicals including xylene, benzene, petroleum naphtha, chlorinated solvents (for example 1,1,1- trichloroethane), polycyclic aromatic hydrocarbons, anthracene, phenanthrene,benzo[a]pyrene, phenols, amines, oil, acids and alkalis (eg sodium hydroxide), polychlorinated biphenyls, halogenated cyanoalkanes, processing aids, and plasticisers. Tyre processing also involves several heavy metals including zinc, cadmium, lead, chromium and copper.

By way of illustration, in one experiment published in 1998 in Environmental Health Perspectives, researchers from the University of Texas’ Houston Health Science Center exposed immune cells called macrophages to ash collected from the Mount St Helen’s eruption and to airborne dust from St Louis and Washington, DC. The volcanic dust had no effect on the cells but the urban dust caused the macrophages, which normally keep immune reactions under control, to die. The likely result of this damage is an overly aggressive immune response, with the potential to cause inflammatory damage to the lungs.

It is hardly surprising, then, to find numerous studies linking the rise in asthma and other allergies over the last 25 years to particulate matter in the air. In the past this problem was blamed on exhaust pipe emissions, particularly diesel exhaust. But now researchers believe that the microscopic dust generated by tyres is equally, if not more, influential. In areas of high traffic it can cause asthmatic attacks and pose a serious threat to elderly patients whose lungs are already weakened by age and illness. Babies and young children are also at increased risk; per pound of body weight, they inhale more particulates than adults.

Studies continuously show that asthma rates are higher in those living close to busy, congested roads. In one 1994 study published in the Archives of Environmental Health, children admitted to one Birmingham hospital for severe respiratory problems were much more likely to be living near busy roads than healthy children or those admitted to hospital for other reasons.


Allergies on the rise

In addition to generalised allergic responses, tyre dust also produces some very specific allergic responses. Tyres are made from a combination of natural latex, derived from rubber trees, and synthetic rubber derived from petroleum. At least 70-75 per cent of all natural rubber produced today is used to make tyres – the rest goes to making latex gloves and condoms, as well as paint and adhesives.

Latex allergy was first noticed in the late 1970s. As the fear of AIDS became a major medical concern, more and more medical workers started wearing latex gloves and this increased exposure to latex meant that allergies became much more common. Today, because of their continuous exposure to latex gloves, tubing and other rubber coated medical and surgical paraphernalia, up to 10 per cent of all health care workers show signs of an allergic reaction to latex.

But studies of individuals living near busy roads has shown that the latex portion of tyre dust can also cause allergic reactions ranging in severity from rhinitis (runny nose) and conjunctivitis (tearful eyes), to hives (urticaria), bronchial asthma, and occasionally even a life-threatening condition called anaphylactic shock.

Equally alarming is the way that latex allergy can produce cross-reactions to a wide variety of fruits, vegetables and legumes. This phenomenon, referred to as latex-fruit syndrome, means that some people who become sensitised to latex will also experience allergic reactions to foods such as banana, kiwi, chestnuts, avocados, peaches, tomatoes, potatoes, peppers and peanuts. Latex-allergic people also occasionally have cross-reactions to some kinds of pollen and herbal remedies. No studies have ever been done that conclusively link tyre dust to latex-fruit syndrome, though it is tempting to speculate that individuals living near busy roads might make a silent, albeit significant, contribution to the increasing prevalence of food allergies in the developed world.


Deadly Dust

The unique combination of known carcinogens, neurotoxins, heavy metals and other poisons in tyre dust can also be linked to more serious diseases. In a 1994 report on the adverse effects of particulate air pollution, published in the Annual Reviews of Public Health, researchers found that for every cubic metre of air, an increase of 20 micrograms (mcg) of particulate matter meant a one per cent increase in deaths from all causes. In this study deaths from respiratory failure, but also heart failure, were much more common as particulate levels increased. This estimate is echoed in the conclusions of a recent report by the non-profit Health Effects Institute in Cambridge, Massachusetts, which found that death rates in the 90 largest US cities rise by 0.5 per cent with only a tiny increase – 10 mcg per cubic metre of air – in particles less than 10 micrometres in diameter.

However, these findings may underestimate the real risk. This month a large, long-term study of residents in the Los Angeles, published in the journal Epidemiolog, found that for each increase of 10 micrograms per cubic metre of fine particles in the neighbourhood’s air, the risk of death from any cause rose by 11 to 17 per cent. The risk of death from diabetes rose more than two-fold and the risk of death from heart disease rose by an astounding 25 to 39 per cent. Similar findings were published in 2003 in Circulation: Journal of the American Heart Association, where researchers looking at 16 years of data on more than a million people concluded that long-term exposure to air pollution posed a greater risk of death from heart disease than it did for death from respiratory ailments.

New research continues to emerge on the mechanics of the microscopic dust-heart failure connection. A healthy heart is able to vary its beats per minute widely, depending on the demands being placed on the body. But when researchers at Harvard Medical School took electrocardiogram (ECG) and blood-pressure readings from 21 Boston residents aged over 50, heart rate variability decreased dramatically when the patients were exposed to air with high levels of very fine particulate matter. Decreased heart rate variability is a known risk factor for sudden heart failure.

Why particulate pollution should have such an effect on the heart rate is still a mystery. One possibility is that when you inhale these very small particles deep into your lungs, some of them make their way into the bloodstream, where they find easy access to organs such as the heart. Once they become lodged in cardiac muscle, these particles may also initiate an inflammatory response that reduces blood flow and speeds the progression of atherosclerosis.

Humans aren’t the only ones affected. While studies into the effects of particulate on animals in their natural habitats are thin on the ground, laboratory studies show that inhaled dust can damage both the hearts and lungs of dogs, mice and other animals. In one investigation, dogs exposed to levels of particulates no higher than those found in many US cities experienced significant changes in their heart rhythms.


Frustration

The impact of particulate matter – from tyres and other sources – on human health is undeniable. It is particularly frustrating, then, that there appears to be no immediate answer to, or recognition of the problem of tyre dust.

In June 2005 a report, Particulate Matter in the United Kingdom, produced by the Air Quality Expert Group, the body that advises government on matters of air quality, concluded that although particulate matter levels have been decreasing over the past few decades, background levels are still very high and more work needs to be done to ensure that national targets for reducing particulate matter are met.

The report also notes that relatively little data exists to quantify the emission rates, size, distribution and composition of particulates from tyre wear. It also reveals that figures for emissions from non-exhaust sources like tyres are not even taken into account in the Atmospheric Emissions Inventory of the UK’s largest city, London. Even without this vital London data, the group estimates that tyre and brake wear emissions accounted for more than 23 per cent of the total road transport emissions.

When government think-tanks think about tyres, they focus on the environmental menace of waste tyres (see box on page 15). This is undoubtedly important, but clearly its scope is woefully inadequate. We’ve got to put the bigger picture of tyre lifecycles onto the agenda before we can even begin to reduce their impact on health and environment.


Spare Tyres

Tyre dust is a significant source of pollution. But what happens at the end of a tyre’s lifecycle produces an altogether different kind of pollution. Piles of waste tyres are rapidly accumulating around the world. As of 2003, about 290 million tyres were discarded in the US every year (roughly one per person). In the UK around 40 million spare tyres accumulate each year. Since 2003 it has been illegal to dump whole tyres in landfills in the UK and by next year it will be illegal to dump chipped tyres into landfills as well. The disposal of tyre waste is now a major problem throughout the world and one to which there are no apparent solutions.

Tyres are designed not to fall apart and this means that they are difficult to dispose of. Although they can remain substantially intact for years beyond their useful life, the number of dangerous chemicals in tyres mean that they can’t be safely burnt. Nevertheless, cement makers and paper mills are happy to use waste tyres as fuel – a disastrous enterprise that produces even higher levels of particulate pollution. Buried in the ground, their constituent chemicals leech out on the ground and water table. Used to make artificial reefs, they can provide homes for certain types of marine life, but are toxic to many fish.

At the moment the best use of old tyres is to extend their life by retreading them. This process involves grinding down the surface, or casing, of the worn tyre until it is smooth and gluing a new veneer of tread onto it. While retreading a tyre uses far fewer resources than buying a new tyre, it is not entirely environmentally friendly since it still involves the use of non-renewable resources to make the new tread, and strong adhesives and other toxic chemicals to attach it to the old casing (and, of course it makes no impact whatsoever on the problem of tyre dust).

A quality car tyre can be retreaded about three times, and tyres for larger vehicles can be retreaded as many as 12 times. Unfortunately, according to the Used Tyre Working Group, a joint industry and government initiative sponsored by the main tyre industry associations, just 18 per cent of Britain’s tyres are retreaded. Around 48,500 tons are converted into crumb rubber – used in carpet underlay and to make springy surfaces for running tracks and children’s playgrounds. However, this makes little dent in the 435,000 tons of waste tyres produced by the UK each year.


Environmental Impacts

At every stage of a tyre’s life cycle, from the sourcing of raw materials to the mountains of waste tyres that blot the landscape, tyres are bad news for the environment. Tyre manufacture relies on fast disappearing natural resources, such as rubber trees, as well as non-renewable resources such as petroleum and mined minerals. It also generates significant levels of localised pollution. Nevertheless, it is during and at the end of their life that tyres generate the greatest amount of harmful pollutants.


Material/Source: Natural rubber. Predominantly obtained from the sap of the Hevea brasiliensi tree.
Application: Currently makes up about 30 to 40 per cent of the total rubber used.
Potential impacts:
■ Loss of habitat in tropical forests.
■ Use of pesticides and hormone-based growth promoters (used
to increase sap production).
■ Transportation to markets.
■ Pollution from rubber processing plants.

Material/Source: Synthetic rubber All synthetic rubbers are made from petrochemicals.
Application: Accounts for approximately 60 to 70 per cent of the total rubber used.
Potential impacts
■ Resource depletion of petroleum.
■ Energy consumption, emissions and waste during manufacture.

Material/Source: Steel cord and beading including the coating materials and activators, copper/tin/zinc/chromium. The steel is premium grade and is only manufactured in a limited number of plants around the world due to the high quality requirements.
Application: Provides rigidity and strength to the tyres. In a car tyre, steel cord makes up about 15 per cent by weight.
Potential impacts:
■ Energy use and toxic emissions during processing and transportation.
■ Difficult to recycle.
■ Leaching of metals during disposal.

Material/Source: Other reinforcing fabrics. Predominantly sourced from petrochemicals.
Application: For structural strength and rigidity. Makes up about five per cent of a radial tyre (one reinforced with steel).
Potential impacts:
■ Toxic emissions/energy use during processing and transportation.

Material/Source: Carbon black. Generally sourced from petroleum stock.
Application: Imparts durability and wear and resistance to degradation. Makes up about 28 per cent of a passenger tyre, although the amount is higher in the rubber that makes up the wearing surfaces.
Potential impacts:
■ Toxic emissions/energy use during processing and transportation.

Material/Source: Zinc oxide. Mined/refined material.
Application: Adds resistance to UV degradation, controls the vulcanisation process and enhances blending. Zinc oxide makes up about 1.2 per cent of a passenger tyre.
Potential impacts:
■ Mining erodes the soil.
■ Toxic emissions/energy use during processing, manufacture and disposal.
■ Leeches into the soil and water from waste tyres.

Material/Source: Sulphur (including compounds). Used to vulcanise the rubber.
Application: Makes up about one per cent of a passenger tyre.
Potential impacts:
■ Toxic emissions/energy use during production/procession.
■ Toxic emissions released when tyres are burned for fuel or disposal.

Material/Source: Other additives and solvents. Age resistors, processing aids, accelerators, vulcanising agents, softeners and fillers are used in the various rubber compounds to modify handling manufacturing and end product properties.
Application: Additives make up about eight per cent of a passenger tyre.
Potential impacts:
■ Various impacts associated with manufacture and transportation including energy use, non-renewable resources and toxic emissions.
■ Impacts associated with use and disposal of the solvents.
■ Emissions from tyres in use, during recycling and in final disposal.



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