For some types of cancer, there were hints that coffee may even be beneficial, said the International Agency for Research on Cancer (IARC)—but not if consumed hotter than 65 degrees Celsius (150 degrees Fahrenheit).

A review of over 1,000 studies concluded that drinking “very hot” beverages was “probably carcinogenic to humans,” said the agency.

“It doesn’t matter what the liquid is. What matters is the temperature,” said epidemiologist Dana Loomis, who took part in the review of the world’s most popular hot drinks.

The IARC looked at the full complement of published scientific literature considering if there was a cancer link to coffee or mate, a South American herbal infusion that is also popular in the Middle East.

Both had been classified as “possibly cancerogenic to humans” since 1991, when the last evaluation was done.

But evidence gathered since could link neither drink to an elevated cancer risk, said IARC, an agency of the UN’s World Health Organization (WHO).

Only when drunk hotter than 65 C did any association arise with cancer of the gullet—the pipe that transports food and fluids from the throat to the stomach, it added.

“Studies in places such as China, the Islamic Republic of Iran, Turkey and South America, where tea or mate is traditionally drunk very hot (at about 70 C) found that the risk of oesophageal cancer increased with the temperature at which the beverage was drunk,” said the IARC.

Some Like it Too Hot

But there was good news for coffee drinkers.

The analysis found that at “normal” temperatures, there was some data pointing to a lower risk of cancer of the uterus, liver and breast.

Studies have also found that coffee had strong antioxidant effects and other possible health benefits.

Taken together, the available data suggests “there is inadequate evidence in humans for the carcinogenicity of coffee drinking,” said the agency.

As for mate—typically drunk piping hot—IARC found it was the probably high temperature, not the drink itself, behind the previously observed link with cancer.

Mate is typically consumed from a closed container with a metal straw that delivers the brew directly to the throat.

“The single study that examined cold mate drinking showed no association with oesophageal cancer,” said the agency.

It also scoured studies on the possible link between cancer and other hot beverages, including milky teas drunk in parts of Africa and central Asia.

Some of the data pointed to “significantly increased relative risks for drinking very hot tea and very hot beverages,” it found.

And in lab studies, very hot water at 65-70 C boosted oesophageal tumours in mice and rats, said the agency.

Way Above Normal

Very hot drinks have thus been classified “probably carcinogenic to humans”, the report concluded.

The IARC—partly due to a lack of research—did not find that drinking very hot water was similarly dangerous.

“It is too speculative at this point,” Loomis told journalists prior to the report’s release.

He stressed that 65 C is “really quite hot”—way above “normal serving temperatures for coffee and tea in European countries and North America,” which are typically below 60 degrees.

The research had taken account of other lifestyle factors that could have skewed the data, such as participants’ alcohol and tobacco use—high risk factors for oesophageal cancer.

According to the WHO, cancer of the food pipe accounts for about 400,000 deaths out of eight million total cancer deaths every year.

This article is from Science’s archive and we’ve originally published it on an earlier date.