At one time it was used as an aftershave because of its sweet smell. It was even used to decaffeinate coffee. Oh my, how times change! Today we worry about a few parts per billion of benzene in our drinking water, in our soft drinks, in our antiperspirants or sun protection products. Why? Because benzene is an established carcinogen and should be avoided. But benzene also happens to be one of the building blocks of our society. Traces of it are everywhere. Given that eliminating benzene from the environment is impossible, what we need is a reasonable risk analysis. That’s quite a challenge, but let’s give it a shot.
Not all appearances of benzene are due to human activity. It is one of the numerous compounds formed when organic matter decomposes, and therefore it can be found in petroleum. It also forms when organic matter burns, so volcanoes and forest fires produce benzene. So does burning coal. In fact Michael Faraday, the brilliant English chemist, first isolated benzene from “illuminating gas” in 1825. Back then combustion of coal or peat produced the gaslight that illuminated homes and streets. The molecular structure of benzene, though, remained a mystery for some forty years. Chemists could not figure out how the six carbon atoms and six hydrogens that made up benzene were joined together. At least not until August Kekule supposedly had a dream in which a snake seized hold of its own tail. This vision led him to suggest a structure for benzene in which the six carbons were joined in a ring. Kekule may have dreamt of a structure for benzene, but he could have hardly imagined the role this compound would play in building the industrialized world.
Today benzene is produced in huge amounts by various processes that rely on petroleum as a feedstock. It serves as the raw material for making plastics such as nylon, polystyrene and polycarbonate, as well as adhesives, detergents, dyes, insecticides, synthetic rubber, explosives and drugs. Without a doubt, benzene makes our life easier, but does it also make it shorter?
The first hint of potential health problems appeared when workers exposed to benzene vapours complained of dizziness, headaches, tremors and even delirium, all symptoms of neurotoxicity. This led to the implementation of measures to reduce benzene exposure in the workplace, but lingering concerns about long-term exposure to small amounts remained. And it turned out that these concerns were justified because epidemiological studies eventually revealed a higher rate of leukemia in workers who inhaled benzene over a period of many years. The connection is not an overwhelming one, it is estimated that since 1928 when the association was first noted, there have been about 150 cases of leukemia worldwide that can be linked to occupational benzene exposure. Estimates are that breathing workplace air at a level of 50 ppb over a 40-year career will result in an increased risk of leukemia.
Most of us don’t have to worry about occupational exposure. But what is the risk associated with benzene that shows up in our food and drink and in the air that we breathe? In theory, no amount of a carcinogen is safe, because a single molecular insult to DNA can lead to cancer. But in practice, elimination of the numerous carcinogens to which we are exposed, both natural and synthetic, is impossible. In the case of benzene, most authorities have set a maximum allowable level in drinking water of 5 parts per billion (ppb). This is not because amounts in excess of this are known to be a health hazard, it is because water treatment systems cannot realistically be expected to reduce levels below 5 ppb.
Now let’s put some numbers into the benzene-cancer equation. We have two sources of information, human exposure data and animal feeding studies. When workers are exposed to less than 0.1 parts per million (ppm) of benzene in the air, there is no evidence of increased risk of leukemia. Since we know the average human inhales about 20 cubic meters of air a day, this translates to exposure to 6 milligrams a day. Animal feeding studies have confirmed that at such doses there is no increased cancer risk. So how much benzene are we exposed to? When it comes to beverages, the issue is the reaction of sodium benzoate, a preservative, with vitamin C, which is present in many drinks. It seems clear that in the presence of trace amounts of metals that catalyze the reaction, vitamin C produces free radicals that can convert benzoate into benzene. That’s why some beverages have been found to contain as much as 50 ppb of benzene, ten times what is allowed in drinking water! But when we make the calculation, we find that a litre of such a drink contains 50 micrograms of benzene, which means that even at an impossible consumption rate of 120 litres a day, we would be below the amount that has no effect on occupational workers.
Of course, soft drinks are not our only exposure to benzene. When the U.S. Food and Drug Administration carried out a survey of 70 foods over five years, benzene was found in every item except for American cheese and vanilla ice cream. A hamburger, for example, has 4 micrograms, but this is only one-tenth of the amount of benzene in the smoke inhaled from a cigarette. A banana can harbour up to 20 micrograms. When you use a deodorant spray or sun protection aerosol you may produce a temporary cloud that contains 28 ppb of benzene. Still, when all exposures are added up, we are well below the levels that have been linked with leukemia.
Is the risk zero? No. In some unlucky person, a trace of benzene may start a cascade of events that leads to cancer. Therefore all efforts should be made to minimize exposure to carcinogens, particularly in the workplace, but traces of benzene in soft drinks do not appear to be a serious risk In any case, these can be eliminated by switching to preservatives other than sodium benzoate. As far as personal care products go, not all deodorants or sunscreens contain benzene so it is certainly possible to produce these without benzene as a contaminant.
If you want to worry about something, worry about the lack of nutrition in soft drinks. Or about the benzene you’re inhaling (20 micrograms) when you’re pumping gas or inhaling city air (25 micrograms per hour) or smoking a cigarette which can expose you to 3,000 micrograms a day. The bottom line here is that manufacturers and food producers should do their best to reduce levels of benzene in their products but in the grand scheme of risks, non-workplace exposure to benzene does not rank very high.
