To Your Health
Is there a concern that cooking-oil vapors could increase the chance of lung cancer?
A. There are many reasons for a higher incidence of lung cancer in China than elsewhere. China is a land where 70% of the men smoke cigarettes. In addition, dust from the Gobi Desert picks up carcinogens, such as dioxin from industrial regions, and this adds to the outdoor air pollution.
In contrast, only about 10% of Chinese women smoke, but they do not enjoy a proportionate drop in lung cancer. The explanation seems to involve indoor air pollution. The high lung cancer rate among rural Chinese women was first blamed on the extensive use of coal in cook stoves, but that did not entirely explain the observations. The incidence of lung cancer was found to increase with the amount of time the women spent preparing meals in a wok, stir-frying at very high temperatures.
The lung cancer risk was also linked with use of rapeseed oil, which is widely used in China though little used in the United States. Tests showed that the oil itself is not carcinogenic but that the vaporized components of rapeseed oil are more carcinogenic than the vapors of other oils. The association of rapeseed oil vapor with cancer generated a storm of controversy, yet to be resolved, over the use of canola oil, since a genetically altered rapeseed plant is used to produce canola oil.
Rapeseed oil has a very high "smoke point," the temperature at which gaseous vapors from the hot oil becomes visible, which probably is part of the reason it is favored in China for stir-frying. Refined canola oil has a similar smoke point but the smoke point of unrefined canola is very low. Refining any oil increases its smoke point but also removes many of the substances that impart unique flavors.
Carcinogens are produced when almost any substance is heated, and in general the higher the temperature the more carcinogens produced. Oils should not be heated above their smoke point. Since this is a marker for the chemical changes that result, not only in reduced flavor and nutritional value, but also in the production of carcinogens, it is important to not inhale these vapors.
Judging the contribution of alcohol intake to the risk of cancer is very difficult, although it has been attempted. In one such list, the part that alcohol plays is considered very significant, comparable to the risk of long-term exposure to formaldehyde and about 30 times higher than the risk associated with a lifetime of drinking coffee. To be fair, many good foods appear on the list with low but measurable levels of risk. Mushrooms and tomatoes, for example, are assigned numbers about 1% of the risk number for alcohol, and substances we normally worry about, saccharine for instance, score the same as carrots. Clearly there is a lot of work to be done in calculating the input of our environment to cancer risk.