Coffee has been downgraded as a possible cause of cancer but drinking "very hot" liquids could increase the hazard, the International Agency for Research on Cancer says.

Coffee dropped in the UN agency's classification of its potential to cause cancer, from "possibly carcinogenic to humans" in 1991 to "unclassifiable" because enough studies haven't been done on humans, a spokesman for the agency said Wednesday.

"There's no particular reason for concern at this stage," Dana Loomis, deputy head of the IARC program that classifies carcinogens, said in an interview. "We can't say that it's completely safe because proving a negative is very difficult, but it has moved down a step in terms of the hierarchy of concern."

To exonerate coffee, scientists would need a substantial body of evidence, he said.

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The best evidence available suggests that coffee does not raise the cancer risk. (Getty Images/Hero Images)

Earlier studies suggesting coffee is associated with bladder cancer were based on less reliable looks at the drink among heavy smokers. Larger, more recent studies in general populations followed over time to see how many cancers develop and how they might relate to coffee don't point to concern, Loomis said.

What's more of a concern is the temperature of hot beverages.

Population studies found the risk of esophageal cancer increased with beverage temperature in China, Iran, Turkey, and South America, where tea or maté — an infusion traditionally made from leaves of trees in South America — are consumed at about 70 C. 

Tongue burns

Coffee is usually consumed cooler than tea and people in North America typically drink beverages closer to 60 C, Loomis added. 

"If you spilled it on your skin, it would hurt and studies that have been done in East Africa where people often drink very hot teas show that people who are drinking it around 70 degrees or more often burn their tongue," he said. 

Several experiments with rats and mice found "very hot" liquids — including water — could promote the development of tumours.

Cancer of the esophagus, a muscular tube that connects the throat to the stomach, is the eighth most common cause of cancer worldwide and one of the main causes of cancer death. The Canadian Cancer Society estimates about 2,200 Canadians are diagnosed with esophageal cancer each year and 2,100 die from it.

"All of us have probably experienced drinking a hot beverage and burning the inside of our mouth. That's usually an indicator, 'I'm not going to drink it. I'm going to wait until it cools down a bit,' and that's the right thing to do," said Robert Nuttall, the cancer society's assistant director of health policy in Toronto.

When hot liquids are consumed regularly, the esophagus can be irritated. increasing cancer risk, Nuttall said. 

Eating red meat and shift work are also in the "probable carcinogen" category, Nuttall said.  

Smoking remains the number 1 cause of cancer.

Nuttall suggests people be active, eat well, limit alcohol intake and be safe in the sun.

In the grand scheme of things, Nuttall reminds Canadians that these other everyday activities contribute to many more cancers than drinking hot beverages.  

The full publication on coffee, maté and hot beverages from the World Health Organization's research arm isn't expected to be published until early 2017 at the earliest. 

With files from CBC's Melanie Glanz