http://www.gardenorganic.org.uk/factsheets/gg2.phpLead and the Gardener
Gardeners and their families are at risk if they grow food in a lead polluted environment. The information collected here draws on research done on lead in soils and lead contamination of food crops, giving recommendations for choice of crop, cultivation practices and careful food preparation that will minimise the risks involved.
Lead in our environment
Lead is a serious pollutant of our environment. Most of it (80%) comes from car exhaust emissions but it can also be released as a by-product of industrial processes. Other sources include residues of lead paint, old lead water pipes, contaminated sewage sludge used as a fertiliser and the use of lead arsenate compounds in orchards. The burning of tyres, soldered tin cans and painted wood also releases lead.
Lead is emitted into the air as particles - the smaller ones remain airborne. We breathe them in; others fall to the ground, polluting soil and dust or settling on trees and plants. Of those we inhale, only the tiniest particles reach the lungs; the remainder are filtered by our nasal passages, but from there get into our saliva and tend to be swallowed.
The majority of the lead we take into our bodies enters through our digestive system, mostly as a result of contaminated food and water. Small children are most vulnerable as they are highly sensitive to lead and also take in lead-contaminated dust and soil licked from their fingers whilst playing.
Low levels of lead (10-20ppm in dry soil) do occur naturally in soils but this is tightly bound and not available to plants. In the industrial world many of our soils already have elevated lead levels. In urban areas or near busy roads, lead particles from the air accumulate in the top 10-20 cm (4-8 in.) of soil, sometimes raising the lead content to hazardous levels (eg. 200-600mg/Kg available lead in central London soils). This has serious implications for gardeners.
Fortunately, in recent years, lead levels in the air have been reduced through the introduction of lead free petrol, a strategy encouraged and supported by increased public awareness of the dangers of lead. Unfortunately, because lead persists in the soil for thousands of years, we now have to live with a legacy of high lead levels in soils that have been polluted.
Why is lead a problem?
Lead is a metal that is toxic to the human body, acting as both a general poison and a nerve poison. Children are particularly at risk: lead interferes with their vitamin D production, and a low level of lead in the environment has been shown to result in impaired mental development in babies and children.
Despite Government/EU attempts to establish safety standards, it is questionable whether there can be a truly "safe" level of exposure to lead because it accumulates in the body and can exert slow, long term effects.
What are the risks from home-grown produce?
The major risk is not that plants will take up high levels of lead through their roots. Plants do not need lead and most species have highly effective natural barriers in their roots to prevent it entering the plant, even when lead soil levels are high as a result of pollution.
Serious lead pollution of vegetables is a problem because of surface contamination. This occurs when lead particles in the air settle on leaves and fruit and become bound to the surface so that they are not readily washed off, and when lead-rich soil adheres to roots and/or splashes onto the crop. At very high lead levels (over 600mg/Kg soil), lead can become toxic to plants and inhibit growth.
What research has shown
Vegetables with a high proportion of the edible part exposed, eg. lettuce and spinach, can accumulate substantial amounts of lead particles from the air, or from soil splashed onto them.
Cabbages grown in urban areas showed highest lead levels in outer leaves, more than three times the level for inner leaves; contamination was also significantly greater nearer to roads.
Root crops such as carrots and swedes developing in contact with contaminated soil will tend to accumulate lead in their skins. This has also been shown for potatoes.
Fruit such as blackberries and raspberries pick up high levels of lead from the air because of their shape - which is particularly efficient at catching lead particles. Blackberries growing alongside busy roads should definitely be avoided as they can pose a serious health hazard.
Onions do not appear to suffer from lead contamination and can be regarded as a safe crop to grow. This would not apply to spring onions as the leaves are consumed and these could pick up lead particles from the air.
What level of lead is acceptable?
There is no single accepted figure for an upper "safe" limit for lead in agricultural or garden soil. The following list illustrates the range:
Department of the Environment 500mg/Kg of dry soil
EU 300mg/Kg of dry soil
Campaign for Lead Free Air 169mg/Kg of dry soil
Soil Association Organic Standards 100mg/Kg of dry soil
In our opinion, you should take special care if growing food crops in any soil with a lead content close to or above 100mg/Kg. Where lead levels are as high as 300-500mg/Kg soil, growing food crops is probably not advisable and it would be unwise to let children play on such soil.
If you are concerned that your soil or produce may be contaminated with lead, get it checked by your local Environmental Health Department.
Alternatively, the Wessex Environmental Consultancy, Bath College of Higher Education, Newton Park, Bath, BA2 9BN (tel: 01225 873701) specialises in soil analysis for lead pollution, and will give advice.