Diesel, cancer and how (not) to report about risk

The BBC reports that diesel exhausts have been declared as causing cancer. That could very well be, but the way the results are reported leaves a lot to be desired. First, look at this bit:

Dr Kurt Straif, also from IARC, said: “For most of the carcinogens when there is high exposure the risk is higher, when there is lower exposure the risk is lower.”

Hmm, so lower exposure to diesel lowers the risk of cancer?! Lower than what? Than no exposure at all? I presume that this is just an awkward way of saying that the risk increases with the amount of exposure. But then at what point does the risk become ‘significantly’ higher than the risk if not being exposed? When you pass behind a diesel car once a day? When you work behind fuming diesel engines all day long? When you get a single overdose once in your life? Without answers to all these questions, the information that some level of exposure to diesel is related to an increase in cancer risk is pretty useless to me.

What is also missing is a crucial comparison to petrol. How does the risk of exposure to diesel compare to the risk of exposure to petrol? Is petrol carcinogenic as well, to a smaller degree, or not at all? Part of the scientific evidence for the carcinogenic effects of diesel is apparently based on observing truck drivers. No details or links to this research are provided, but I wonder how these truck drivers have been isolated from the effects of petrol fumes and exposed to the effects of diesel fumes only…

The most disturbing part comes with the statement that ‘Diesel exhausts are now in the same group as carcinogens ranging from wood chippings to plutonium and sunlight to alcohol.’ Plutonium and alcohol! So is inhaling diesel fumes carcinogenic to the same extent that drinking a glass of wine once a week is, or to the extent that ingesting plutonium is?

I certainly understand the scientific rationale to classify diesel fumes as carcinogenic. What I don’t understand the journalistic rationale to report this without the necessary context to make the information understandable and useful for the ordinary readers. After all, shall we all sell our diesel cars now? I mean, diesel fumes do cause cancer, as it says in the title, no?

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