At one time it was used as an aftershave because of its sweet smell. It was even used to decaffeinate coffee. Clementine Churchill (Winston's wife) washed her hair with it. Oh my, how times change! Today we worry about a few parts per billion of benzene in our drinking water or in our soft drinks. Why? Because benzene is an established carcinogen, and should be avoided (or so say the likes of the Environmental Working Group). 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 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.
But most of us don't have to worry about occupational exposure. What is the level of 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 an 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? Recently we heard about a concern about benzene in soft drinks. At issue is the reaction of sodium benzoate, a preservative, with vitamin C, which is present in many beverages. It seems clear that in the presence of trace amounts of metals which 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 liter of such a drink contains 50 micrograms of benzene, which means that even at an impossible consumption rate of 120 liters a day, we would be below the amount that has no effect in 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 seventy 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. 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 are not a big deal. Even these could in principle be eliminated by switching to preservatives other than sodium benzoate, but if you want to worry about something, worry about the lack of nutrition in soft drinks. Or about the benzene you're inhaling when you're pumping gas. That's about 20 micrograms -- but even if you let the gas station attendant pump your gas, you'll be breathing in about 20-30 micrograms per hour just from the exhaust of the cars in front of you. In comparison, the average total daily intake from diet is about 5 micrograms.
Such an analysis also holds true for the other volatile organic compounds (VOCs) which are of concern in our environment. When FDA carried out its study of the food supply, it investigated the presence of over twenty other VOCs which could be expected to show up because of their widespread use as solvents, cleaning agents, degreasers, or intermediates in various chemical processes. Some could even enter the food supply as byproducts of water chlorination or as migrants from plastics. Accordingly, chemical analyses were carried out for the likes of styrene, chloroform, carbon tetrachloride, and trichloroethylene, all of which are suspected carcinogens. All of these were found to be present at the ppb level, but like benzene, the amounts were well below toxic levels.
As an example, for carbon tetrachloride, the minimum risk level has been established at 0.02 mg per kg per day. Some carbon tetrachloride can be detected in hot dogs at 11 ppb. A 70 kg man would have to eat 120 kilos of hot dogs a day to reach this level. Basically then, all of these chemicals are inhaled at much higher doses through cigarette smoke, gasoline fumes, or industrial emissions. So if you really want to achieve zero risk, you can eat and drink without worry, but just be sure not to breathe.
Dr. Joe Schwarcz is Director of the McGill University Office for Science and Society.
For more ACSH items on benzene and EWG's irrational campaign against it, see:
"Benzene Redux: This Time It's Soda"
"FDA Unafraid of Benzene in Soda"
"Memo to FDA: Take the Toxins Out of Our Food!" (A Parody)
