A reason to wash your vegetables

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  • Published: Mar 21, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: Gas Chromatography
thumbnail image: A reason to wash your vegetables

Poisonous pesticides

Pesticides are a cornerstone of modern agriculture. Without them, we would struggle to feed the growing population. However, their unregulated or excessive use can have negative consequences, including pollution of the environment.

Pesticides are a cornerstone of modern agriculture. Without them, we would struggle to feed the growing population. However, their unregulated or excessive use can have negative consequences, including pollution of the environment. A less recognised threat, however, is that to human health. Pesticides can contain toxins and potential carcinogens that can accumulate in the body. Evidence suggests that this accumulation (even at very low concentrations) might damage the reproductive and immune systems, and contribute to neurodegenerative diseases and cancer – especially when vegetables are eaten fresh.

Although there are international regulations to protect the environment from the effects of pesticides, there remains concern about human exposure. The effects on people may be less noticeable than those on wildlife, as people are exposed to lower concentrations and gradually, over long time periods. Key to better understanding the risk, and as a result developing regulations that protect human health, is knowing exactly how much pesticides are left on vegetables after harvest.

Fortunately, modern analysis techniques, such as gas chromatography and mass spectrometry, can sensitively and efficiently detect pesticide residues. A combination of the two (GC-MS) is particularly effective, providing high-resolution qualitative and quantitative information. In a study recently published in Biomedical Chromatography, researchers used this approach to analyse pesticide residues in vegetables from Western China.

Assessing the market

The researchers assessed a total of 51 regularly used pesticides in 369 different vegetables, including cabbage, cauliflower, mushrooms, lettuce, cucumbers and tomatoes. The vegetables were collected from local markets before being analysed by GC-MS.

The first pesticide group the researchers looked at was organophosphates, which are widely used to boost yields and reduce post-harvest losses owing to their potent ability to kill insects at low cost. At least 33 thousand tonnes are used each year in the US alone. Their mechanism (which involves blocking an enzyme essential for nervous system function) is not specific to insects however, and may be harmful to brain development, even at low concentrations.

Levels of these pesticides ranged from 0.0008 mg per kg to 8.82 mg/kg. In eight samples, concentrations exceeded maximum residue levels (MRLs) – the highest level that is legally tolerated in food. These samples could potentially be unsafe for human consumption.

Carbamate pesticides have a similar mechanism of action to organophosphates and have been used since the 1950s, popular owing to their broad spectrum of action. Similarly, organochloride pesticides also target the nervous system and include compounds like DDT, which is now banned worldwide owing to its toxic effects on wildlife. Concentrations of these pesticides ranged from 0.012 to 0.79 mg/kg. Organochloride MRLs were exceeded in four samples, while carbamate MRLs were exceeded in six.

This is particularly concerning, as organochlorides do not break down easily in the environment and can accumulate up food chains, making them especially toxic. Indeed, the researchers found evidence of two organochloride pesticides (BHC and carofuran) that were banned in China over 40 years ago.

Finally, the researchers looked at levels of pyrethroid pesticides, used in the majority of household insecticides and generally considered safer than other pesticides. The researchers analysed five types of these pesticides, which were detected at levels up to 6.08 mg/kg. Although these pesticides were found at higher concentrations than the others, and two samples exceeded safe levels, they are also less toxic. The researchers say the high concentrations found were likely due to their stability, low toxicity and cost compared to more traditional organophosphate pesticides – which has led to widespread use.

Exceeding safe limits

Although the majority (70%) of vegetable samples were completely safe and no pesticide could be detected at all, the remaining 30% contained at least one type of pesticide residue. Some contained multiple residues and several exceeded safe limits.

The study highlights a human health concern that requires greater attention. As well as encouraging governments to tighten regulation on pesticides, the researchers recommend that farmers use pesticides that are less toxic, both to the environment and people. Finally, they say routine monitoring of vegetables – using techniques such as GC-MS – is important to minimise the health risks.

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