Thanks to scientists working under the auspices of the World Health Organization, you can be fairly sure your toothbrush won’t give you cancer. Over four decades, a WHO research agency has assessed 989 substances and activities, ranging from arsenic to hairdressing, and found only one was “probably not” likely to cause cancer in humans. It was an ingredient in nylon used in stretchy yoga pants and toothbrush bristles.

All the other 988 substances, however, pose some level of risk or need further research, according to the International Agency for Research on Cancer (IARC), which is an arm of the WHO. Some things in IARC’s top category of carcinogens are pretty obvious nasties, such as plutonium, mustard gas and smoking tobacco. Others are more surprising: Also ranked as “Group 1 Carcinogens” are wood dust and Chinese salted fish.

IARC has said that working as a painter causes cancer, using a mobile phone possibly does, and working shifts as a pilot or a nurse, for example – is “probably carcinogenic.” Last October, it ranked processed meats in its top category of known carcinogens, alongside plutonium.

The findings have caused consternation, not least for non-scientists puzzled by what IARC’s rankings mean.

As a global authority on cancer – a disease that kills more than 8 million people a year worldwide, with more than 14 million new cases appearing annually – IARC has enormous influence and commands much respect, even among its critics. Yet experts from academia, industry and public health say IARC confuses the public and policymakers. Some critics say the way IARC considers and communicates whether substances are carcinogenic is flawed and needs reform.

Even the WHO, which oversees IARC, was caught off guard by the agency’s announcement that red and processed meat should be classified respectively as probable and known carcinogens. The WHO’s official spokesman, Gregory Hartl, issued a statement saying WHO’s Geneva headquarters had been flooded with queries and requests for clarification. IARC’s ruling did not mean people should stop eating meat, he said.

Asked about the relationship between IARC and the WHO, Hartl told Reuters: “WHO works closely and continually with IARC to improve the way the two bodies collaborate and communicate on the knowledge of potential and real hazards and risks to the public.”

At stake are judgments that can affect the lives of millions of people and the economic activities of states and multinational companies. IARC’s rulings influence many things, from whether chemicals are licensed for use in industry to whether consumers choose or spurn certain products or lifestyles.

But its methods are poorly understood and do not serve the public well, according to Bob Tarone, a statistician formerly at America’s National Cancer Institute and now Biostatistics Director at the International Epidemiology Institute. He said of the way IARC works: “It’s not good for science, it’s not good for regulatory agencies. And for people? Well, they are just being confused.”

Paolo Boffetta worked at IARC for 19 years, rising to become head of the genetics and epidemiology team, and describes himself as “still a strong supporter” of the agency. Nevertheless, Boffetta, now at the Mount Sinai School of Medicine in the United States, said IARC’s approach sometimes lacks “scientific rigour” because its judgments can involve experts reviewing their own research or that of colleagues.

Read more on Reuters Investigates website