George Washington reputedly once commented that he would rather be on his farm cultivating food than be the emperor of the world. It is likely that the people gardening the 300,000 or so allotments in the UK would understand that sentiment, not to mention the vast number of people who would like to join them. But Washington never had to consider the risks caused by pollution from passing cars. Paul Landau reports.
The UK has a long history of allocating small patches of land to the public for growing food; indeed, every citizen has the right to an allotment, and every local authority has the duty to provide them. Despite some ups and downs in allotments, there is now resurgence in home-grown food, often known as grow-your-own (GYO). Moreover, allotments and vegetable gardens are often located in urban areas, and while GYO provides many health benefits, the urban environment is one with a legacy of pollution, especially contaminated land. Therefore, many studies have looked at pollutants such as heavy metals in GYO fruits and vegetables, focusing on levels of contaminants in soils and the potential risks that they pose.
Far fewer studies, however, have examined the potential impacts of air pollution on GYO plants, especially in areas close to roads. So a group of researchers from the Ukraine and Germany1 examined the concentration of heavy metals in a variety of crops grown in Berlin, examining how the locality affected the findings. The researchers found significant differences in heavy metal concentrations, which in turn were influenced by the plant species, style of planting, proximity to traffic, and the presence of buildings or other barriers. In simple terms, more traffic nearby resulted in higher concentrations of metals in plants, while barriers such as buildings or other structures could reduce the relative concentrations. This article examines the impacts of traffic on GYO vegetables and fruits, and explores what this means for urban agriculture and the risks to human health.
The growth of GYO
According to a survey from 2011 conducted by the National Allotment Association (NAA), there are over 3500 allotment sites in England alone, comprising over 150,000 plots. In the UK, it is estimated that there could be twice as many allotments2. At the same time, the NAA survey noted that there were around 90,000 people on waiting lists for allotments, often with waiting times of several years or more.
There have been many studies which have shown that the pastime of growing fruit and vegetables can have significant benefits for both physiological and mental health. For example, GYO is likely to increase the consumption of fresh fruit and vegetables at a time when there are increasing concerns about obesity, lack of exercise and poor diets. In terms of psychological health, allotments and vegetable patches have been shown to provide significant benefits for people who live in cities and other highly urbanised areas, especially for those who do not have a garden or access to one.
However, this in itself poses a potential environmental risk. As allotments are typically in the urban environment, while most food consumed by people is growing in rural locations, it is implicit that GYO vegetables are more likely to be exposed to pollutants. Notably, many allotments are in areas that are close to either existing or former industrial areas, with still many more near to major roads.
Considering that there are legally binding limits for allowable trace levels of substances, such as pesticides and heavy metals in food sold to the public, many researchers have investigated whether there are such contaminants in GYO food. A large number of investigations have focused on the presence of heavy metals in soils, and the uptake of these contaminants from soils. For example, investigations worldwide have looked at concentrations of metals such as lead, cadmium and arsenic in both soils and plants, attempting to see whether common risk assessment methods for soil pollution correlate with the levels of contaminants found in plants. A study in England examined whether the national Contaminated Land Exposure Assessment (CLEA) model was a reliable measure of the safety of GYO plants, while an investigation in Chicago focused on lead pollution. Both studies found that risk assessment methods for soil pollution are not always reliable indicators of the concentrations of contaminants in food, and nor do they provide a consistent method for determining the risks to human health.
However, far fewer studies have examined the air as a potential and actual pathway for contaminants in food grown in the urban environment, and most of the investigations that have been carried out were carefully controlled, experimental studies. Therefore, a team of researchers decided to see how GYO fruits and vegetables fared when grown on typical allotments and vegetable gardens in an inner city. So what did they find?
Sources and measurements
In the investigation, a team of researchers from the Department of Ecology at the Technical University of Berlin and the Botanical Garden of the Khmelnitskij National University in the Ukraine focused on metals in the edible parts of plants downwind of traffic. Their aim was to determine the risks to human health. The team had examined research from the 1970s, which suggested a close link between the distance of plants from roads, and the accumulation of trace metal pollutants such as lead, zinc and cadmium. The concentrations of pollutants were influenced by the volume of traffic, wind direction and speed, length of exposure and any barriers between the plants and roads, such as trees.
The team then built on this work, looking at different species and styles of planting; for example, whether the plants were grown in beds of soil beds or pots. There were 28 randomly chosen sites in the study, with varying characteristics in terms of traffic burden and barriers. The plant types, however, were the same throughout the sites, to provide a good comparison between locations and styles of planting. The plants included: tomato, green beans, carrot, potato, kohlrabi (German turnip — a type of cabbage), white cabbage, nasturtium, parsley, chard, basil, mint and thyme. In order to set a baseline for contaminants, the team also analysed plants taken from either supermarkets or rural locations. After sampling the plants, each type was analysed for several heavy metals, which included zinc, cadmium, chromium, copper and nickel.
The concentrations of contaminants varied widely according to species of plants, but there was not a significant difference between the types of plants. The largest influences appeared to be due to the style of planting, the traffic burden and the presence of barriers.
When compared to the same species grown in rural locations, the plants grown in the city had concentrations of metals many times higher, especially for lead in the edible parts of all plants. When plants were grown in areas within 10 metres of a major road, about two thirds of the plants had levels of lead above the EC standards. In contrast, this fell to about a third for plants grown more than 10m from the nearest street. At the same time, growing plants behind a barrier, eg a building or tall vegetation such as trees, could halve the number of plants with lead levels over the EC limit. The study did not report other metals above the EC limits, although the team was surprised to find that plants grown in pots could have higher concentrations of metals when compared to those grown in beds of soil.
In terms of impacts on health, the situation is more complex. While urban plants have higher concentrations of metals than rural plants, there is a consensus among researchers that the health benefits of GYO fruits and vegetables are likely to outweigh the potential risks to health — for example, eating fruits and vegetables containing trace metals above the recommended limits is not necessarily harmful unless someone eats massive amounts of such plants. At the same time, current exposure assessment models for soil contamination can produce a simplistic view of the health risks.
Lastly, the team from Germany and Ukraine clearly recognised the benefits of GYO, and advised that if someone wishes to grow edible plants in a city, then it is best to do so away from a road. And if that is not possible, then a barrier between the road and the plants will significantly reduce the risks of metal contamination.
“How Healthy is Urban Horticulture in High Traffic Areas? Trace Metal Concentrations in Vegetable Crops from Plantings within Inner City Neighbourhoods in Berlin, Germany”, in Environmental Pollution 165, Säumel, I et al, 2012
“Health Benefits of “Grow Your Own” Food in Urban Areas: Implications for Contaminated Land Risk Assessment and Risk Management?”, in Environmental Health 8, Leake, J et al, 2009
Last reviewed 20 August 2012