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Food Focus June 2009

Is it safe? A risky calculation

Food contamination on a large scale can happen at anytime. Sometimes the culprit is microbiological – caused by bacteria or viruses – or chemical. Chemical contaminants in food can be naturally-occurring, like tutin and cyanide, or man-made, like melamine. When chemical food contamination occurs, NZFSA toxicologists conduct a risk assessment to calculate the possible risk to the health of anyone who eats the contaminated product. The answer is rarely clear cut, but the public want certainty in the advice they’re given about food safety.

New Zealand Food Safety Authority (NZFSA) toxicologist John Reeve says consumers’ perceptions of risk from food are often not the same as the risks identified and assessed in the objective science-based processes used by regulators.

“This makes it difficult to deliver clear messages. Scientists and regulators consider food okay to eat when the levels of hazards, like chemical residues, are either not present at all or are within acceptable limits. Consumers find it difficult to accept that safety is anything other than black or white.”

Everything including the kitchen bench

In February 2009, NZFSA was notified that an imported iron supplement used to fortify some milk products was contaminated with the industrial chemical melamine.

Just months earlier some babies had died and thousands had reportedly been made very sick following deliberate criminal adulteration of milk powder in China, so of course alarm bells were ringing. This time, there was no criminal adulteration – the contamination was accidental due to a processing glitch in the overseas factory where the ingredient was made. Tests at an approved laboratory showed that due to dilution of the ingredient in processing, the melamine would not even be detected in the final product – mostly milk powder which was then itself used as an ingredient in products like biscuits. So, was this latest event even a safety concern at all?

Initial tests of two samples of the iron supplement ingredient showed they contained residues of melamine in excess of the 5ppm levels for food ingredients which prompts the involvement of New Zealand’s food safety regulator, NZFSA. Melamine is not a permitted additive in food and there are no safe levels set for it.

Melamine is widely used in plastics, adhesives, benchtops, dishware, whiteboards, flame retardants and fertilizers. Food safety regulators now recognise that incidental contamination is unavoidable through leaching into food from processing equipment and packaging materials. As a result, the World Health Organization has set a tolerable daily intake (TDI) for melamine. The TDI, which is very similar to an acceptable daily intake or ADI, represents the amount of a substance in food that a person can ingest on a daily basis for a lifetime without showing any health effect. The TDI allows national authorities to set limits in food that would spark a withdrawal of a food product should melamine be detected as a result of intentional adulteration.

So, when melamine was found in an ingredient for food products that would take time to confirm had not already reached shop shelves, the question everyone wanted the answer to was: “if I have eaten any, is it safe?” To find out, John Reeve ran a quick risk assessment based on a worst case scenario which assumed all foods containing the iron supplement were contaminated by melamine at its detection limit, even though none had been detected in testing.

“We take a very conservative approach when it comes to food safety. It’s highly unlikely that a chemical found to be safe would turn out not to be, while something thought to be unsafe could prove to be okay at low levels. So we always calculate risk based on the highest possible dose of toxin consumed by the most sensitive individual.”

In this case, John says, it was a relatively straightforward calculation.

Safety as the sum of all things ÷ 100

The toxic effect of eating melamine-tainted food is fairly well known as a result of data collected following a melamine-tainted pet food scandal in the United States in 2007, then the 2008 adulteration of milk with melamine in China. From this information and the test results, John had some certainties as a starting point for his calculation.

John knew the highest level of melamine likely to be found in the iron supplement ingredient. He knew how much of the iron supplement ingredient was used in the food. He knew how much of the final food product a person would be likely to consume, based on national nutrition surveys which supply a wealth of useful detail about what we eat, how much and how often. He also knew from animal studies the highest dose of melamine at which there is no observable adverse effect (NOAEL) on the body, and that melamine doesn’t accumulate but is excreted fairly rapidly through urine.

As well as considering what we know for certain, the calculation also factors in all those things we don’t know, aren’t sure about, or which vary from individual to individual. These ‘uncertainty factors’ include: the difference in the effects of the toxin on laboratory animals for which we often have accurate data and humans, for which we may not; varying ages, body weights and health of consumers; whether the available data is complete; and the quality of the available data which can also be variable.

Traditionally, the dose just below where an effect can be seen is divided by uncertainty factors in order to build in an acceptable level of safety. The calculation always starts by dividing by two 10-fold uncertainty factors to account for the fact that usually the available data applies to animals rather than humans, and for differences between individuals. That means assessment of the risk starts with estimating a level of contaminant a person needs to consume that is more than 100 times less than that we would expect someone to consume before showing any observable adverse health effect. Further uncertainty factors (UF) are then used as appropriate.

From this information John could calculate the potential health risks to anyone eating food containing the contaminated ingredient. On the basis of assuming the worst, he calculated the highest possible amount of melamine that a susceptible person might eat each day.

“In this case, by comparing the highest possible intake of melamine with the acceptable intake we were able to say with certainty that the melamine in the imported ingredient did not pose a health risk. But it shouldn’t have been there. People often think that if it shouldn’t be there and there are no safe levels set for it, then it’s dangerous but it doesn’t always work like that.

“The definition of ‘safety’ in these sorts of incidents is our best judgement about the level of exposure to something that has a reasonable certainty of no harm to humans. People take risks of varying sorts every day. We drive cars, cross busy roads with earphones impairing our awareness, use electrical equipment, smoke cigarettes. It’s how we manage those risks that’s important.”

Managing food safety risks is another story, John says.

“We try to keep risk assessment and risk management separate. As someone who assesses risk I have to be completely focussed on what the science is telling me is safe or not safe. It’s up to the risk managers to then balance the science and other factors, such as what people believe to be an acceptable level of risk, to come up with strategies or tactics for ensuring that any risk is acceptable while being fair and necessary.”

What does toxicological risk assessment involve?

Toxicologists are the science world’s jacks of all trades, according the NZFSA toxicologist John Reeve.

“Toxicologists have to have a good understanding of the life sciences: pharmacology, biochemistry, genetics, physiology, and so on, because many of the systems affected by toxins are inter-related or interdependent. Changes to one organ can cause changes to other systems or parts of the body.”

Toxicological risk assessment involves biological and systemic detective work, tracking and understanding changes in the body possibly caused by toxins, rather than the ‘normal’ fluctuations or unrelated events that occur from time to time in the body’s systems.

“We’re all subject to daily ups and downs of moods or physiological responses. These reactions might be due to a toxin, or they may be blips that are entirely unrelated and natural. The trick is working out which is which.”