Proposition 65, otherwise known as The Safe Drinking Water and Toxic Enforcement Act, is making more impacts in California. A judge has recently ordered that coffee companies will be required to put warning labels on their cups declaring that coffee carries risks of cancer. While many coffee shops do already have warning signs in their store, this law would require warning labels in plain sight. This is an example of a scientific question influencing policy and seems to be surrounded by misunderstanding so here I want to address why coffee is considered by some to be dangerous and how well that perspective falls in line with what experiments have been able to demonstrate. Coffee is getting this attention because the beans, like some other starchy foods, form small amounts of acrylamide after being heated to high temperatures (250F). The amount of acrylamide in any heated food varies depending on how long this chemical reaction goes on. But for coffee, the average numbers are as follows: https://www.ncbi.nlm.nih.gov/pubmed/24325083 Since we do not know the minimum dose that can translate to an increased risk of cancer in humans, these numbers are hard to interpret on a practical level. Nevertheless, acrylamide is listed by the International Association on Cancer Research (IARC) as a "probable carcinogen." Since there is always confusion about what this actually means whenever it is reported, let's take the time to clarify this. Whenever a substance is classified by IARC, both lab and human studies are taken into consideration. In the case of acrylamide, lab studies that involve putting it in rats' drinking water (in amounts equaling 1,000 to 10,000x the amount humans are exposed to) have shown that acrylamide greatly increases risk of certain cancers. Human studies have tried to correlate acrylamide with cancer risk by measuring how often subjects consume foods higher in acrylamide. https://www.cancer.org/cancer/cancer-causes/acrylamide.html While these studies are survey based and thus have limitations, they have not revealed an increased risk in any cancers associated with acrylamide. So while animal studies show that acrylamide can cause cancer, no evidence suggests that it poses a danger at the levels found in cooked foods. These two sets of data lead to the classification of "probably carcinogenic." As I've explained in other threads, this classification does not speak to how carcinogenic the chemical can be, just whether or not it is a carcinogen in the context of normal exposure. Given this information, we now have the question: should people be warned about this? How worried should people be that probable carcinogens are in their coffee? The reality is that we are exposed to a variety of carcinogens every day just by eating, drinking, and breathing air. Whether or not these exposures pose a risk to our health is determined by lab experiments, human case studies, and looking at trends in human populations. Typically, warning labels are placed if evidence is strong in all of these categories and a reasonable causal link can be established. This was the case with cigarettes. One can argue that just because we don't yet have the evidence, we can say that people have the right to know that they are consuming something that might be harmful. However, preemptively labeling goods as such can cause a lot of unnecessary confusion and worry. While it is always possible that next week some clever group of researchers might find a causal link between the amounts of acrylamide consumed in coffee and certain cancers, the fact that such a link has not been obvious up until now is a good indication that the link would very likely be weak, if it exists. The evidence to justify a cancer warning label on all coffee cups because of acrylamide is just not there right now.