Researchers have suggested guidance in the United Kingdom should be revised because it does not recognize how lead is stored in soil and transferred to vegetables.

Recent years have seen a rise in popularity of people growing their own fruit and vegetables, but many urban allotments have soil with lead levels above U.K. guidance values.

The study, published in the journal Environment International, highlighted suitability of certain crops for growing at sites with elevated lead such as shrub and tree fruit, whilst limiting consumption of seleced root vegetables, such as rhubarb, beetroot, parsnips and carrots.

Variation of lead uptake in the different crops highlights a role for focusing on the types of crop grown as a way to mitigate any raised exposure. Management practices include keeping soil moist during dry periods and in windy conditions; peeling and thoroughly washing all crops, and hands, before eating.

Lead concentration in soil above guidance
The Environment Agency introduced guidelines in 2014 stating that 80 milligrams of lead per kilogram of soil was considered safe. Transfer of lead to humans can occur via direct exposure to soil and indirect exposure such as eating food containing lead and contaminated water.

Newcastle City Council previously investigated allotments and found the average lead level was 550 mg per kilo. The council commissioned a study to investigate whether the level of lead in soil was reflected in the levels in blood of gardeners. They would be exposed to lead whilst gardening and from eating the vegetables they grow, meaning they could potentially face health issues.

Academics from Northumbria’s Department of Geography and Environmental Sciences and Newcastle University worked with the council, U.K. Health and Safety Laboratory, ALS Limited, the Food Standards Agency and Environment Agency on the two-year Newcastle Allotments Lead Biomonitoring Study.

The team examined almost 300 samples of soil and crops from 31 sites in Newcastle. They found lead concentrations within vegetables grown in the allotments – such as parsnips, carrots, leeks and onions – varied depending on the vegetable, but was typically below national food safety guidelines.

In 98 percent of sampled soils, lead concentrations were above U.K. soil guideline. Researchers did not find statistically significant differences in blood lead levels after analyzing blood from more than 43 gardeners compared to 28 of their non-gardening neighbors.

Professor Jane Entwistle, an environmental geochemist at Northumbria University, co-led the study.

“Many urban allotments in the city have higher concentrations of lead in their soil as the land was previously used for the disposal of hearth ash, in addition to the build-up of generations worth of urban air pollution from traffic emissions,” she said.

Entwistle suggested the soil screening guidelines have overestimated the amount of lead going into plants.

“U.K. soil screening guidelines are overly cautious in the context of many of our urban allotments and our research has found that how the lead is held in the soil is more important than simply the presence of lead. While some metals are more readily taken up by plants, lead remains attached to the soil as it is not a mobile element. What we therefore need to do is look at the form of the lead and natural regional variances rather than using standard guidelines that don’t recognize these differences,” she said.

Lead health risk
Allotment soils in Newcastle had lead which had aged and weathered naturally over the years, while the guidelines were developed based on a range of soils, some of which had been artificially contaminated with lead in a lab, or wher it had come from wastewater and sewage treatment areas.

Lindsay Bramwell, from Newcastle University, said lead can cause health issues including high blood pressure, reduced fertility and anemia.

“Gardening and eating food grown in contaminated urban areas can increase our exposure to lead. Fortunately, our study found that this is not the case and has, in fact, helped to explain why there is no difference in blood lead levels in gardeners compared to non-gardeners.”

The study found fruit and vegetable consumption rates by all participants to be considerably higher than those used to derive the UK’s category 4 screening levels. The focus was on adults but scientists recognized the need to consider exposure of children to lead in future work.

Researchers said the guidelines should be revised depending on the type of soil and how tightly the lead is bound within it.

“Allotment gardening has always been considered a healthy and beneficial activity in terms of physical and mental wellbeing of individual allotment gardeners and for the social health of their local communities. These study findings give allotment gardeners peace of mind that they can continue their passion for gardening without the worry that the site will be closed because of unfounded health concerns,” said Mark Todd, Newcastle City allotment officer.