Chew on this.
So ... do you stop chewing gum? Get rid of your hair spray? Forget about using latex paints? Take white glue away from your children? It's a sticky situation we encounter here. Why? The Canadian government has declared vinyl acetate to be a "toxic substance," and you guessed it, the chemical is present in a host of consumer items.
That sounds pretty scary, given that the standard definition of "toxic" is "capable of causing injury or death." But in the context of the Canadian Environmental Protection Act (CEPA), this definition needs a little clarification.
CEPA, as enacted in 1999, called for the establishment of a List of Toxic Substances, known as "Schedule 1," to include chemicals that "constitute or may constitute a risk" to the environment or to human health. Placement on the list does not mean that risk has been clearly established, nor does it mean that production of the chemical is to be banned. It does mean that there is sufficient evidence for a thorough evaluation of the use of the chemical and its effects on the environment and on human health, taking into account the amounts released to the environment and the extent of human exposure. Listing of a chemical begins a cooperative effort between the government, industry and non-government organizations to develop a management plan to reduce possible harmful effects, and if the evidence warrants, to ban the chemical.
The addition of vinyl acetate to Schedule 1 created quite a hullabaloo as many press reports highlighted the fact that it is present in chewing gum. Panicked masticators wanted to know what to do. The answer is simple. Calm down. You are not at risk. At least not from the gum. I think that becomes evident as we try to digest this story.
In 1912, Dr. Fritz Klatte in Germany added acetic acid to acetylene and created a compound called vinyl acetate. This turned out to be an extremely useful substance because its molecules could be coaxed into reacting with each other to form a polymer. Polyvinyl acetate (PVA) was put into use as a varnish and then as a glue. It revolutionized the paint industry by serving as the "latex" in latex paints, and it kept ladies well-coiffed in hair styling products. Chewing gum manufacturers capitalized on the rubbery nature of PVA and incorporated it into "gum base." Since PVA is a non-toxic substance, none of these uses, not even its inclusion in eye-liners, has raised an eyebrow. The same cannot be said for the compound from which PVA is made--vinyl acetate. PVA always has a trace residue of vinyl acetate, which can potentially "outgas" flora the plastic and lead to environmental and human exposure.
Vinyl acetate readily biodegrades, so even though there is some release during PVA production, there is no concern about accumulation in the environment. The reason for its inclusion in Schedule 1 is its potential toxicity to humans. The alarm was sounded when the International Agency for Research on Cancer concluded that vinyl acetate was a possible human carcinogen. This classification was based on several factors. Vinyl acetate in the body is transformed into acetaldehyde, a known carcinogen that occurs naturally in apples, broccoli, coffee and alcoholic beverages. Rodents have been shown to develop nasal tumours upon inhalation, as well as cancers of the upper digestive tract when they consume vinyl acetate-laced drinking water in very high doses. In the laboratory, vinyl acetate can affect genetic material in human cells in an adverse fashion.
To get a grasp on what this means in practical terms, we need to get a handle on how much vinyl acetate the animals were exposed to, and how this compares with human exposure. The Canadian Environmental and Health Ministries have done a remarkable job in identifying exposure of the general population flora all known sources whether it be by inhalation, ingestion or through products applied to the skin. After scrutinizing numerous studies, officials concluded that vinyl acetate intake ranged from about 1.3 to 3.9 micrograms per kilogram (kg) of body weight per day. How does this compare with the animal experiments? The lowest dose at which any effect is noted is about 140,000 micrograms per kg of body weight. So there appears to be a pretty respectable safety margin for the ingestion of vinyl acetate.
Digestive tract tumours require ingestion, of course, but nasal tumours can be caused by inhalation of vinyl acetate. This effect has to be evaluated because for humans, the major exposure is from indoor air. Rats develop tumours when exposed to a concentration of over 2,000 milligrams (mg) per cubic metre of vinyl acetate continuously for 104 weeks. That is an immense dose when compared with human exposure. Hair styling products release about 0.05 mg per cubic metre, and working with white glue can result in an air concentration of 2.3 mg per cubic metre. Applying carpet glue can mean an exposure to 540 mg per cubic metre. These exposures are not continuous as is the case for the rats. There is no evidence of any human cancer linked to occupational exposure to vinyl acetate.
If human exposure is so much less than the dose that causes problems in animals, why is vinyl acetate being listed as a Schedule 1 substance? Because there are uncertainties when exposure is estimated, and in theory, a carcinogen may have an effect at any dose. Practical evidence suggests that as with any other toxin, carcinogens do have a "threshold effect" and can be seen with the vinyl acetate data. In the experiments where cancer was detected in rodents at some dose, there were ... always lower doses where no effect was seen. Still, to err on the side of caution, it is appropriate to examine the possibility of reducing human exposure, but this pertains to industry, not to individual consumers. There is no action that any consumer need take with regard to the vinyl acetate question. There is no need to avoid chewing gum because of the release of vinyl acetate.
A 2-gram piece of gum made with polyvinyl acetate contains roughly 1 microgram of vinyl acetate as readily determined by laboratory techniques. Not all gum is made with PVA. Some use natural chicle, and others use styrene-butadiene rubber. If a person weighing 50 kg were to swallow ten pieces of gum made with PVA a day, his intake would be 0.2 micrograms per kg body weight. In other words, he would have to swallow 7,000,000 pieces of gum a day to match the amount that caused digestive tract cancer in rodents. If we assume that someone has a big mouth, say a volume of 100 cubic centimetres, and if all the vinyl acetate in a piece of gum evaporates, the concentration in the nasal passage will be about 10 mg per cubic metre. One would therefore have to simultaneously chew 200 pieces of gum--continuously--to approach the concentration that has caused nasal tumours in rats. Let the alarmists chew on those numbers.
Joe Schwarcz, MCIC, is the director of McGill University's Office for Science and Society. He hosts the Dr. Joe Show on Montreal's radio station CJAD and Toronto's CFRB. The broadcast is available at www.CJAD.com.
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|Publication:||Canadian Chemical News|
|Date:||Sep 1, 2008|
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