What is a “chemical” after all?
My toxicology professor John Doull from Kansas City, KS, once wrote: "Even Rachel Carson was made of chemicals." He was clearly sick and tired of the view held by the media that chemicals are strange artificial substances jeopardizing our health. One example of such thinking was demonstrated by an organization measuring environmental chemicals in commissioners of the European Union and ministers of member countries. Several newspapers publicized how 38 chemicals were in this Commissioner or as many as 55 chemicals had been detected in some European politicians.
There are two different misunderstandings in that sort of news. A more sensitive analysis would have revealed all studied 103 chemicals in all of the participants. In other words, the number of chemicals is totally useless information, because it only reveals the detection capability of the laboratory used. Professor Jaakko Paasivirta from Jyväskylä, Finland, used to say in his lectures that he could easily tell the brand of mosquito repellent used by the person who had picked the berries he was analysing. This kind of sensitivity is in fact not yet even close to the sensitivity needed in analysing dioxins in normal people or mother’s milk.
Another mistake is that there are not a hundred or even ten thousand "chemicals" or any other fixed number of substances. As John Doull stated, everything is chemicals and there is nothing but chemicals in the world. You cannot escape chemicals, you yourself are made of chemicals. The entire world is a mixture of chemicals, and only a small proportion of chemicals in our environment are man-made.
The chemicals of our environment are rarely pure, but human activities may produce large amounts of pure chemicals. Water or hydrogen oxide is a pure chemical we use in the largest amounts. Nature keeps distilling water all the time, when it evaporates and falls back as rain. Regular tap water is 99.9 per cent pure water. Mineral waters contain more than 0.1 percent of salts and much of the time also carbonic acid. Therefore they are less than 99.9 percent pure water. Sugar is 99.9 percent pure sucrose, it is a purer chemical than many laboratory chemicals, to say nothing of technical chemicals. In fact, sugar is one of the chemicals produced in the largest volumes in pure form. There are some other pure chemicals known to us – salt, soda, acetone and ether. Many medicines are chemicals isolated and purified from nature such as digitalis, penicillin, atropine, morphine, and levodopa. Many of them can also be made by chemical synthetic processes.
Most substances in nature around us are mixtures of chemicals. The fats, proteins and carbohydrates in our food are not often thought of as mixtures of chemicals, but that is what they in fact are, and there is no difference in principle between substances naturally in the environment and those made by humans.
There are, however, some differences in practice between chemicals made by nature and those made by man. Life produces chemicals from elementary units following several relatively simple rules. Plants synthesize sugars by using two substrates, carbon dioxide from the atmosphere and water. Green chlorophyll of plants utilizes the energy of sunlight for photosynthesis, and helps enzymes to synthesize first low-molecular weight intermediates such as organic acids. These are then used to construct more complex organic molecules such as sugars, fatty acids, and also amino acids by adding nitrogen from soil nitrates.
These building blocks are then used to construct large macromolecules, carbohydrates such as starch or cellulose from sugars, proteins from amino acids, and fats and waxes from fatty acids and alcohols like glycerol. In addition, different small and medium size molecules are constructed; these are sometimes very poisonous. A common feature of the natural chemicals is that they can usually be broken down to small-molecular forms, often all the way back to carbon dioxide. In other words, they can be "burned" or oxidized. Organisms have the enzymatic machinery both to build and to break down organic compounds.
Problems of synthetic substances
Many of the early synthetic products were like natural organic products in that the enzymatic machinery of our body could handle them in the same way as the natural compounds. Often they were in fact "semisynthetic" compounds, being synthesized from natural starting materials. Examples are tar, ether, methyl alcohol or ammonia. They can be easily handled by biological mechanisms, because their parts are familiar to the body’s machinery.
With the advances in chemistry, man learned to make artificial substances with no counterparts in nature. Many of them contained a halogen such as chlorine, often also fluorine. It is often typical of these chemicals that microbes, animals or humans cannot break them down with their enzymes. An extreme example is dioxin (TCDD), the half-life of which is almost ten years in human beings, because it is neither broken down nor excreted. It is a very resistant and poorly water-soluble substance, and even soil bacteria succeed in breaking it down extremely slowly. Therefore it resembles some persistent metal elements such as cadmium which once they gain access to the body, are almost impossible to remove.
The difference is not toxicity but sometimes persistence
Both natural compounds and man-made substances include very poisonous and quite harmless chemicals. Natural chemicals can usually be broken down, while some synthetic compounds are very persistent. This was initially thought of as an advantage. PCB oils seemed to be very good for heavy duty use in difficult conditions. They are heat-resistant, not flammable, insulate electricity, and resist heavy pressure. Therefore they were used in hydraulic pumps and electrical equipment, until it was realized that the apparent advantage was, in fact, a disadvantage. These chemicals are not broken down, and therefore they gradually accumulate in the environment.
The whole world is built of chemicals. There is no reason to be afraid of chemicals, but it would be very useful to understand them and the rules of their behaviour.
Notes and references
- Enzymes are proteins that direct chemical reactions to construct molecules (e.g. sugar from acetic acid) or break down molecules (e.g. sugars to carbon dioxide).
- See the chapter "Isn’t it safe to eat everything that grows naturally?".
- Microbe is a general name for organisms seen only under the microscope or electron microscope. Important microbes include bacteria, fungi and viruses.
- See the chapter "Are the dioxins the most dangerous chemicals in our environment?".
One level up: Perception of the risks around us
Next chapter: Is it more dangerous for us than for our ancestors?