Diet Coke addicts had a nasty shock in June as headlines exploded with warnings the World Health Organisation was about to classify aspartame - a common sweetener - as "possibly" carcinogenic. 

On July 13, the International Agency for Research on Cancer (IARC) - a branch of WHO - duly did so, upgrading aspartame from a category Three to a category 2B cancer risk. 

This is an eclectic field which so far includes 323 agents ranging from talcum powder and "traditional Asian pickled vegetables" to bracken fern, extremely low frequency magnetic fields, and carpentry/joinery. 

Before we go any further, however, it is worth unpicking how the IARC system actually works. 

There are four carcinogen rankings: Group One (tobacco smoking, UV radiation, and other things known to cause cancer); 2A (red meat consumption, night-shift working, and other things which "probably" increase cancer risk); 2B (things which "possibly" cause cancer), and Three (unknown cancer risk). 

What is important to understand is that these classifications are not based on the degree of risk, but the strength of the available evidence. 

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In other words, the classifications do not tell us how dangerous something is - only the level of certainty we have that they are dangerous at all. 

That means something proven to increase the risk of cancer substantially (such as cigarettes) could sit alongside something in Group One that had been shown to increase it only a negligible amount (eating bacon, for example). 

Groups 2A and 2B, meanwhile, have been described as an "epidemiological dumping ground" where current evidence is weak and unclear.

One of the main studies which contributed to aspartame's "possibly" carcinogenic rating was an observational study published last year in the journal PLOS Medicine based on cancer incidence in around 102,000 adults who were followed up between 2009 and 2021.

It found that the overall risk of cancer was 13% higher among those with above average aspartame intakes compared to those who consumed none, after adjusting for variables including age, sex, smoking and weight.

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However, the study does not prove cause-and-effect.

It is unclear how accurate the intake estimates were (they were calculated using self-reported food diaries) and the results may be skewed by other confounding factors.

For example, those consuming higher levels of aspartame also consumed more highly-processed food and sugary drinks.

It is also worth noting that the increased risk only translated into an extra three cancer cases per 10,000 adults over an eight year period.

This is one of the major stumbling blocks we face when trying to understand health hazards: the difference between relative, and absolute, risk.

Bowel cancer is a good example.

The Herald:

In 2015, the WHO re-classified processed meat as a Group One carcinogen, sparking some fairly frightening media claims that eating bacon and hot dogs was "almost as bad as smoking" (a misunderstanding of how the classification system works) and increased the risk of developing bowel cancer by 18%.

The latter figure is correct, but it can also be a bit misleading.

It is based on UK data estimating that a person's chances of developing bowel cancer in their lifetime ranged from 56 per 1000 for those who ate the least amount of processed meat to 66 per 1000 for the heaviest consumers.

If you divide those 10 extra cases by 56, and multiply by 100, you get 18% - the relative risk.

But the absolute risk of developing bowel cancer in your lifetime if you eat a lot of sausages, ham, and bacon is actually increased by only 1%: the extra 10 cases divided by 1000.

Imagine another scenario, where the lifetime risk of developing Disease X is 10 per 10,000 people, but is elevated to 15 per 10,000 among people who are exposed to Product A (a drug, food additive etc).

The relative risk is 50% higher among people using Product A.

Scary stuff; but much less daunting when you consider that absolute risk has only increased by 0.05%.

The Herald:

This latter figure is "undeniably a lot less frightening", noted Dr David Robert Grimes, a cancer researcher, in his 2019 book 'The Irrational Ape'.

He added: "The way in which probabilistic data is reported has a huge effect on how we understand it, and our emotional processing of that information.

"This is especially true of information pertaining to our health and mortality...relative statistics always sound more stark than absolute numbers, despite reporting the same information."

You can find similar conundrums for lung cancer and smoking.

It is proven beyond doubt that regular exposure to tobacco products - both directly and through second-hand smoke - causes cancer.

An estimated 86% of lung cancer deaths and 72% of lung cancer cases in the UK are linked to smoking.

According to Cancer Research UK, the lifetime risk of developing lung cancer is 26 times higher in men who smoke 15-24 cigarettes per day compared with people who have never smoked.

Yet it is also true that 80-90% of smokers never develop lung cancer.

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This puzzle was summed up by Dr Donald Hensrud, a physician specialising in nutrition at the highly-acclaimed Mayo Clinic in the US, when asked to comment on the aspartame decision. 

He said: "First of all, the level of risk is low, but consuming something over many years, even in a low amount, may possibly add to the risk...a definite carcinogen would be cigarettes.

"Cigarettes are known to cause lung cancer. However, we also know that not everyone who smokes gets lung cancer.

"Even with a definite carcinogen, there's some element of uncertainty with regard to risk."

The Herald:

For its part, the WHO stresses that more research is needed on aspartame and that "safety is not a major concern at the doses which are commonly used". 

Besides Diet Coke and other low-calorie sodas, aspartame is found in low fat yoghurts, sugar free jelly, and even toothpaste. 

But you would have to be consuming a lot to exceed WHO's "acceptable daily intake" of 40 milligrams per kilogram of body weight. 

For a 60kg (132Ib) female that would be roughly eight to 12 cans of diet soda per day, every day. 

As ever, though, if you want zero risk - drink water.