Credit: Alchemist-HP
We probably all come into contact with lead in the course of our lives. It is as ubiquitous as its usefulness suggests. Lead is used in car batteries, weight belts for diving, weights for lifting in exercise equipment, ammunition and even radiation protection.
But it’s also potentially deadly if ingested. And here’s the rub. In addition to its usefulness in our day-to-day lives, lead has a history of being used in other ways that have resulted in dispersing it into the environment. Chief among these as an additive to petrol and paint.
It is a non-essential element and toxic to humans as well as animals through the food chain and soil dust inhalation or ingestion.
Indeed, lead has a long history of use in human activities, dating back to Roman times when it was used in pipes to carry water. There are suggestions that the fall of the Roman Empire is at least partially due to the long-running effects of prolonged lead poisoning.
More recently, lead was a popular ingredient in cosmetics in 16th and 17th century Europe because it helped produce the very pale complexion popular at the time. Its users included Queen Elizabeth I who would doubtless have been better off avoiding it.
In England there are a number of historical lead mining areas - the Derbyshire Peak District for one - where there is a legacy of lead contamination resulting from mining and related activities.
What is lead?
A lustrous silver-blue heavy metal, lead is solid at room temperature and occurs naturally in trace amounts on earth’s surface environment. For the scientifically minded, its symbol is Pb (based on the Latin word plumbum, or "waterworks," in reference to Roman times when lead was used for water pipes) and has an atomic number of 82.
Concentrations in rocks average about 15 mg kg. Most acid igneous rocks (such as granites) have higher lead concentrations than basic ones such as basalts. Concentrations in sedimentary rocks vary with up to 70 mg kg in some limestones.
It is corrosion resistant making it useful in lining tanks for corrosive materials like sulfuric acid.
How does lead get into soil?
The Environment Agency, Defra and the Health Protection Agency have published an extensive briefing note on lead and its dangers to human health. They state unequivocally that soil “is a significant sink for anthropogenic lead” and lists “several well recognised major sources: mining and smelting activities; sewage sludge usage in agriculture; and aerial contamination from vehicle exhausts… Historically, lead arsenate may have been applied to orchard trees to control pests and such soils may contain small amounts of lead residues.”
They add that “human exposure to inorganic lead occurs primarily through food and drinking water, although exposure via soil, dust, air and paint chips significantly contribute.”
The World Health Organization added a word of caution, stressing the particular vulnerability of children to lead poisoning “because they absorb 4–5 times as much ingested lead as adults from a given source.
“Moreover, children’s innate curiosity and their age-appropriate hand-to-mouth behaviour result in their mouthing and swallowing lead-containing or lead-coated objects, such as contaminated soil or dust and flakes from decaying lead-containing paint.”
What about the Germans? Was the Blitz really responsible for lead soil contamination in the UK?
At the risk of re-enacting a Fawlty Towers skit, the fact is that recent research suggests there is indeed a connection between the mass bombing of cities in Britain during the Second World War and elevated levels of calcium, zinc and, yes, lead in soil.
British Geological Survey researchers concluded that elevated levels of all three in parts of London were caused by the distribution of dust and debris that was the result of the mass destruction of buildings during the Blitz.
The elements were widely used in paint, mortar and piping in construction during the previous century.
A heat map Britain showing the concentration of bombing raids during the war, illustrates just how widespread the problem is:
What is lead poisoning and what are the symptoms?
Many instances of lead poisoning are due to ongoing low-dose exposure. Because its symptoms are predominantly emotional and mental in nature, lead poisoning is not easily spotted. Children are at greatest risk because lead can delay physical and mental development in babies and the young. It’s also dangerous to adults with accumulations in the body resulting in kidney and nervous system damage, anaemia, stroke and cancer.
According to a Penn State University report: “Serious human health risks, particularly for children under 6 years of age, are associated with lead poisoning… Low-level, chronic exposure to lead in contaminated residential soil can cause several developmental and behavioral problems in children. Among these are reduced IQ and attention span, hyperactivity, impaired growth, learning disabilities, hearing loss, and insomnia. Once absorbed by the human body, lead is extremely difficult, if not impossible to remove. Therefore, not only is prevention of lead poisoning the best cure, but it may be the only cure.”
What are the acceptable levels of lead in soil?
This is a question with several answers, depending on which authority one is inclined to follow. In the UK, so-called Category 4 Screening Levels (C4SL) for Lead have been published - these differentiate between land uses. FYI, lead concentrations in soil are measured in parts per million or milligrams per kilogram (mg kg).
The soil guideline values (SGV) are as follows:
Residential (with consumption of homegrown produce) 200
Residential (without consumption of homegrown produce) 310
Allotments 80
Commercial: 2330
Meanwhile the EPA in the United States uses a threshold of 400 mg kg for residential soil while the SGVs replaced by the C4SLs was 450 for all residential uses and allotments and 750 for commercial.
How can I test for lead in soil?
The Safe Soil UK Standard Contamination Test suite measures the amount of lead and a host of other heavy metals and common contaminants in the soil of your growing space.
Is lead contamination definitely a threat?
Recent research in the UK suggests that the picture may be more complicated than initially believed.
A study published in Environment International found some crops are more suitable for growing at sites with elevated lead – these include shrub and tree fruit – while others like beetroot, parsnips and carrots are less so.
Newcastle City Council tested the soil in its allotments and found the average lead level to be around 550 mg kg. Rather than remediating or closing the sites, the council commissioned a study to look into whether elevated lead levels in soil were reflected in the levels of lead in the blood of gardeners, who were exposed to the lead in the ground and presumably consumed the produce they grew on it.
No elevated blood levels were found though the numbers participating in the study were in the dozens suggesting more research is necessary before the suggested limits for lead in soil are relaxed.
Northumbria University environmental geochemist Professor Jane Entwistle was co-lead on the study and said: “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 recognise these differences.”