Leaving soft drinks in warm conditions, such as a car boot or garage, can significantly increase the chance of benzene forming in the drink, said a scientist who tested the effects of heat and light on benzene in soft drinks for the industry in 1990.
Recent tests by UK and US food safety watchdogs have found several soft drinks containing benzene traces above the countries' respective limits for drinking water. The suspected source was two common ingredients in the drinks - sodium benzoate and ascorbic acid (vitamin C).
Benzene is a listed carcinogen, although both authorities said the levels found in drinks to date should not pose an immediate health risk.
One scientist who helped the soft drinks industry sort out the same problem in 1990, however, said testing drinks after exposure to heat and light was now crucial.
"When those 38 drinks that [the UK Food Standards Agency] tested positive for benzene are subjected to even short periods of heat and light, they could dramatically increase to beyond the WHO 10 parts per billion water standard."
He and New York-based lawyer Ross Getman, who have pioneered the re-emergence of the benzene in soft drinks issue, said food safety watchdogs should make sure they expose drinks containing sodium benzoate and ascorbic acid to heat.
Britain's Food Standards Agency has not tested soft drinks for benzene after heat exposure; although a European Commission spokesperson said new guidelines on benzene testing, now being drawn up by the soft drinks industry, were likely to include "predictive testing to simulate storage".
Industry testing on soft drinks 15 years ago is thought to have found that temperatures of 30°C and exposure to UV light for several hours were enough to more than triple benzene residues in some drinks.
The tests were designed to simulate the worst case scenario, and "were not necessarily representative of what the consumer was receiving", according to Greg Diachenko, a scientist with the US Food and Drug Administration, who also took part in negotiations with soft drinks makers over benzene in 1990 and 1991.
Data reported by America's soft drinks industry association in the 1980s, however, showed that soft drinks could be exposed to between 32°C and 49°C in US summer months.
The association said hot warehouses and cars parked in direct sunlight were examples of when soft drinks would be exposed to higher temperatures.
"Heat is a major factor" in the formation of benzene in drinks, according to Mike Redman, a scientist with the American Beverage Association and who also represented the industry in meetings with the FDA over benzene in 1990.
Redman told BeverageDaily.com that soft drinks firms reformulated drinks in 1990, mainly by adjusting the levels of sodium benzoate and ascorbic acid, to reduce and control the potential for benzene traces to form.
Still, the continuing presence of soft drinks containing benzene above drinking water standards has led to calls for sodium benzoate to be taken out of drinks formulas.
"What are we to tell consumers? 'Product contains cancer-causing substance, drink immediately, do not store in a warm environment or near sunlight?' Preferably benzoate should not be used in combination with vitamin C (ascorbic acid) or added juice," said the scientist involved in industry testing for benzene 15 years ago.
BeverageDaily.com broke the current story on benzene in soft drinks, after it uncovered in February that recent FDA testing had found some drinks containing benzene above America's limit tap water.