For many years, we have heard a good deal about the economic and health threat of pesticides in meat. This is the tip of an ever-growing iceberg. Our widespread use of chemicals foreign to the human body, xenobiotics, is a problem that could reach catastrophic proportions unless we learn to live with them sensibly.
It is axiomatic to declare that almost everything we do is in some way risky. Jogging, swimming, eating, smoking, driving a car, sitting in the sun, just being alive involves us in a risk. We try to protect ourselves from the ultimate danger of death. In our minds we weigh the risk of our actions against the anticipated benefit.
Not often do we dwell heavily upon the risk of something or even its severity. Our actions are tempered by our perceptions, perceptions that are moulded by experience and by how the risk is portrayed by others. Most often these days it is the news media that have the greatest impact on risk perception.
The Seveso disaster is a case in point. How many people died of dioxin poisoning after the explosion of a chemical reactor in that northern Italian in July, 1976? The answer is none. Nor were there any foetal abnormalities or spontaneous abortions that could be attributed to this undoubtedly highly toxic chemical.
But what of the toxic cloud of dioxin (2,3,7,8-Tetrachlorodibenzodioxin or TCDD) that spread over and contaminated 700 acres of land? Without question it constituted a grave hazard and did give rise to cases of the severe skin complaint chloracne, but most skin lesions were caused not by dioxin but by droplets of caustic soda and corrosive sodium phenate thrown out during the explosion.
Dioxin was not the immediate principle ogre of Seveso, but the Press worldwide led us to believe that it was. Now we are permanently prejudiced in our view of the accident. The acute effects of dioxin were not dramatic, but it will be many years before we realise the chronic consequences. As with Chernobyl, in the future there is likely to be a higher level of cancer in those contaminated.
Seveso is an example of how our views are formed, but it also illustrates the way in which risks and benefits are balanced till something goes tragically wrong. Was it a reasonable risk to have a potentially lethal chemicals factory so close to people’s homes? Clearly the city fathers believed that the jobs and wealth the factory brought were worth it, but our views change dramatically with time and are tempered by experience.
A more recent chemical-plant explosion, in 1984 at Bhopal, India, caused the deaths of about 8000 people and resulted in respiratory damage to thousands more. On this occasion the culprit chemical was methyl isocyanate used at the Union Carbide plant for the manufacture of the pesticide Sevin.
Unlike at Seveso and other plants in Europe, where dioxin was an anticipated by-product, there has not been a major outcry aimed at closing such factories. The Indian Government is calling for improved plant design and safety measures to prevent accidents in the future. India has judged that it needs desperately the employment and wealth that the chemical industry brings.
Tragic and dramatic accidents are obvious to everyone. We are immediately aware of a problem but probably not of its long-term consequences. But there is the insidious threat of what we cannot see – effects that chemicals in small amounts can have when they linger or build up in the natural and living environments.
Dangerous depletion of the ozone layer that protects us from ultraviolet light, caused by chlorofluorocarbons and chlorobromocarbons from aerosols and fire retardants, and destructive acid rain in Europe and North America are examples of this. Nearer to home we are experiencing the consequences of chemical contamination of food.
There are so many ways in which we are willing to accept the benefits of chemical technology without weighing seriously the risks. The desire to maximize productivity in agriculture and to improve the appearance and quality of the product has recently put a large chunk of Australian exports in jeopardy.
Producers not only use chemicals to combat diseases in animals and crops but to improve yields. Animal growth promotants have been around for 30 years or more. Antibiotics, other antibacterials and hormones are very effective at increasing weight gain in pigs, poultry and cattle.
A different problem is encountered with antibiotics. Although antibiotic residues do occasionally turn up in meat and milk, the main concern is the possible development of resistant bacteria, particularly salmonella, bugs responsible for outbreaks of food poisoning.
Exposure of bacteria to subtherapeutic amounts of antibiotics can cause the build-up in bacteria of resistance not just to one antibiotic but to many. The bacteria have the ability to transfer the resistance they have acquired to their neighbors. This multiple cross-resistance renders cases of food poisoning caused by these salmonella difficult to treat. In the US more than two-thirds of antibiotic-resistant salmonella infections have been traced back to food animals.
The danger of the overuse of antibiotics in agriculture was recognized in Britain in 1969 with the publication of the Swann report. The report recommended that antibiotics used in human therapy should not be used in animal feeds for the purposes of stimulating growth. Here the benefit was clearly outweighed by the risk, but even so cases still arise where Swann is ignored.
Pesticides pose quite a different problem. Again, they can be used therapeutically for the elimination of parasites like ticks and blowfly, and when used sensibly, according to the manufacturer’s instructions, no serious residue problems need occur. But some insecticides, the chlorohydrocarbons, including DDT, Dieldrin, Aldrin and Heptachlor, have been used for the intensive spraying of crops.
These chemicals, particularly DDT, suffer from the grave disadvantage of high stability; they are not easily degraded in the soil and some can persist there for many years, even decades. From the soil they can pass to pasture and from pasture to cattle and sheep where they concentrate in fat.
They are biomagnified. If the pasture levels are high enough, the pesticide will be detectable in meat. This phenomenon has been known for many years and was the subject of a severe warning in 1962 by the biologist Rachel Carson in her book Silent Spring.
It was the potential economic impact of bans on Australian meat exports to the US and possibly other countries, and not the distant danger to human health, that alerted us to the seriousness of the problem.
There is so little chance of acute or short-term toxic effects from the contamination as to be ignored, and comments about the number of tonnes of meat someone would have to eat to suffer ill-effects are fatuous. They beg the important question of the likelihood of chronic or long-term consequences of pesticide residues to this and future generations.
Our quality of life and longevity are due largely to modern medicines, improved hygiene, and better nutrition. It is hard to believe that of the medicines we use today most have been with us for fewer than 50 years. We are only just beginning to learn of their idiosyncrasies.
You might ask, “What about all the tests drugs and chemicals go through before they are allowed on to the market?” Toxicology is only as good as the state of the art. Medicines are subjected to a barrage of animal tests, tests for lethality, carcinogenicity and teratology (birth defects).
If the drug jumps the hurdle it may be used in humans. But mice are not humans and neither are monkeys, and people differ enormously. So how good is toxicological testing? Animal and in vitro tests do give some idea of possible harmful effects, but they will never be foolproof. There is no such thing as a safe medicine and society will have to grasp that.
Now we must use technology to aim our medicines and thereby reduce side-effects, and hope that educators will pay far more attention to pharmacology in the medical curriculum. In turn the public has a responsibility to understand far better the consequences of their every action – taking a tablet, spraying a crop or squirting an aerosol can is potentially hazardous.
Litigation is the order of today. If something goes wrong, someone has to be blamed. There is an unwillingness to accept that we bear some accountability for our own actions. We can choose not to accept the benefits of medicines, pesticides and aerosols.
As researchers delve deeper into the world of chemistry and chemicals, society can expect to reap the benefits of better health, quality food and prolonged life. These benefits, however, will always leave the occasional risk of a Bhopal or thalidomide.
Professor Parfitt was Deputy Vice-Chancellor (research) of the University of Western Australia.