Introduction

Environmental heavy metal contamination, especially by lead in soil and sediment, has become increasingly recognised over the last 40 or more years as a significant problem in public health, and in the United Kingdom (UK) there is now comprehensive and complex environmental legislation and associated guidance. This paper is a critical assessment of one national directive: the UK Department for Environment, Food and Rural Affairs (DEFRA): document SGV10 “Soil Guideline Values for Lead Contamination” (DEFRA 2002). The contradictions in this document reveal a lack of dialogue between the scientific and regulatory communities.

Lead pollution from petrol in England was first reported by Davies and Holmes (1972), and many authors have since documented the problem. The use of tetra-ethyl lead as an anti-knock agent in internal combustion engines has been discontinued in many countries (Nriagu 1990). The problem of metal-contaminated English garden soils was shown by Warren and Delavault (1971) and confirmed for lead by Davies (1978); the literature on urban residential land contamination is now voluminous. In western and northern areas of Britain there are many derelict base metal mines, and past mining activities have contaminated agricultural land especially floodplains (Alloway and Davies 1971). Application of sewage sludge to agricultural land has caused widespread, usually low level, metal contamination; it was first noted by Le Riche (1968) when zinc toxicity symptoms were seen in vegetables in a long-term manurial experiment.

In the 30 or more years since those early reports there has been intensive environmental research in the British Isles, and the relationship between lead in soil and health has been firmly established. Hands dirtied by soil or dust, or direct mouthing of soil (pica) are now understood to be an important pathway for lead from the environment to the body, and lead in home grown vegetables may also contribute. Comprehensive laws to discontinue the use of materials containing lead and regulate the use of contaminated land have been enacted in the UK.

In the 1980s, the need to identify injurious concentrations of lead in soil, dust or sediment became urgent, together with a recognition that pathways for lead needed both quantifying and modelling. One response to these perceived needs was the setting up of an international task force, under the auspices of the Society for Environmental Geochemistry and Health (SEGH), to review lead in soil and recommend Soil Guideline Values to protect the health of children (Wixson and Davies 1993, 1994). The guideline values were derived using an empirical relationship (see below) to compute target soil lead concentrations from blood data in the context of a phased action plan for systematically investigating and confirming soil pollution.

In the UK, DEFRA together with the UK Environment Agency has published (DEFRA 2002) document SGV10 “Soil Guideline Values for Lead Contamination”. This sets out the derivation of Soil Guideline Values for lead contamination and is intended to be of use by all parties involved with or interested in soil contamination. DEFRA (2002) bases its recommendations on the SEGH model and sets a limit of 450 mg Pb kg−1 soil for residential land (with or without plant uptake) as protective for children and 750 mg Pb kg−1 for commercial or industrial environments.

Although the need to set guidelines to protect child health is unquestionably necessary it is important to recognise that adverse societal consequences may arise from inappropriate standards. The population of England and Wales (some 60 million in 2005) is primarily urban (only 6% live in open rural countryside) and urban garden soils are commonly contaminated by lead (Culbard et al. 1988). The level at which a soil concentration standard is set will determine how many urban gardens may need remediation. If remediation involves excavating the soil this is likely to be both disruptive and expensive. The subsequent disposal of the soil raises the possibility that it might need costly handling as hazardous waste. The same considerations apply for land contaminated by mining and industry. Guideline values need to be adequately protective of child health yet not impose unwarranted societal penalties.

This paper reviews the DEFRA choice of default values for the variables in the SEGH model and examines some practical consequences that flow from them. It is intended neither as a comprehensive review of the relevant scientific literature since 1993 nor an overview of British regulations. Rather, the aim of the paper is a cautionary one, namely that environmental legislation should be subject to ongoing review by the scientific community, including the source of the data underlying the legislation, the provider of new research data and stakeholders in environmental health protection. Without such feedback, legislation may be defective or oppressive.

The SEGH task force and its recommendations

By the mid-1980s, it had been accepted in the scientific community that lead in soil and dust was an important contributor to blood lead especially in small children (toddlers). In 1988, an international conference (Davies and Wixson 1988) was held in the USA (Chapel Hill, NC) to examine the issue of lead in soil and the possibility of framing guidelines for use by national regulatory agencies. The recommendation from that conference was that SEGH should organise a task force to review and advise on the issue. Major objectives were seen as developing a general concept and quantitative framework from which to devise standardised protocols. It was noted that no single soil lead value would probably be applicable to all sites and each site should be evaluated individually according to available information and local abatement requirements. The consequent task force report was published in 1993 (Wixson and Davies 1993) and summarised by Wixson and Davies (1994).

The task force proposed a model to relate soil lead to blood lead and represented it by the empirical equation:

$$ S = 1000\left[ {\frac{{(T/G^n ) - B}}{\delta }} \right] $$

the variable and their numerical values are explained below.

The variables in the SEGH equation

Soil Guideline Value

S is the soil or dust guideline value, a geometric mean concentration in mg Pb per kg of soil or dust (mg kg−1 or ppm); it is calculated from the equation above. The variables in the equation are these:

Background blood value (B)

B is the background or baseline blood lead (BLL) concentration in the population from sources other than soil and dust. Modern data for lead in the blood of the UK population do not appear to have been published. The US Centers for Disease Control and Prevention (CDC) in Atlanta, GA, monitors blood lead levels (BLLs) in US children (CDC 2002). The current situation in the USA is shown in Table 1. Present US background values are the outcome of many years effort to remove or minimise lead derived from paint or gasoline (Chisholm 2001).

Table 1 Blood lead level (μg Pb dl−1 blood) measurements for children aged 1–5, United States (Source CDC 2002)
Full size table

Blood lead guideline value (T)

T is the blood lead guideline or target concentration, in μg Pb dl−1 whole blood.

Geometric standard deviation of the blood lead distribution

G is the Geometric standard deviation of the blood lead distribution, typically in the range of 1.3–1.5.

Slope or response of the blood lead–soil (dust) lead relationship

The slope or response of the blood lead–soil (dust) lead relationship (δ or delta) has the units of μg Pb dl−1 blood increase per 1,000 mg kg−1 increment of soil or dust.

The task force noted that studies by Bornschein et al. (1989) indicated a delta of about 2, while Rabinowitz and Bellinger (1988) reported a value of 0.9 for well maintained middle class neighborhoods. Marcus and Cohen (1988) suggested a value of 2 as the most likely value, this being the median value reported in the papers reviewed. While values 2–5 would be appropriate for most situations the value used should reflect the particular situation. In particular, a well maintained vegetative cover indicates a low value of delta. The SEGH task force recommended using lower values of delta in most situations.

Protection of the population

Parameter n is the number of standard deviations corresponding to the degree of protection required for the population at risk, and would normally follow from the way in which the blood lead guideline T was defined.

  • 95%, n = 1.645

  • 98%, n = 2.05

  • 99.9%, n = 3.04

A 95% protection level was deemed appropriate in most circumstances.

The DEFRA implementation of the SEGH model

Child health

DEFRA (2002) document bases its guideline model for relating soil lead concentrations to child blood lead on the SEGH task force formula. Default values are proposed for the several parameters required in the computation. The values chosen by DEFRA in SGV10 are given in Table 2.

Table 2 Default values for variables in the SEGH equation cited in SGV10
Full size table

The blood lead guideline or target concentration T (μg Pb dl−1 whole blood) was set as T = 10. CDC (2002) interprets the value 10 μg Pb dl−1 as a threshold for identifying “elevated vales” of blood lead levels (BLL); it is a “level of concern” (Federal Register 2001) and identifies a “lead poisoned child”. This is often misread as an environmental intervention value. For values >10 and <14 μg Pb dl−1, the main CDC requirement is follow-up blood monitoring. While extensive interventions are not necessarily recommended, children with blood lead concentrations at or above 10 μg dl−1 require more frequent rescreening at minimum, and some may require environmental or medical interventions. In addition, if many children in a community have blood lead concentrations above 10 μg dl−1, community-wide intervention activities are recommended.

For values 15–19 μg Pb dl−1 health intervention is required if, after three months, values remain within this range or increase. When the environmental lead hazards have been eliminated, the child’s BLL has declined to below 15 μg dl−1 for at least 6 months, and other objectives of the plan have been achieved, the case should be closed (CDC 2002). Medical and environmental interventions are recommended for all children with blood lead concentrations at or above 20 μg dl−1. The clear inference to be drawn here is that BLL = 15 μg dl−1 is an appropriate threshold at and above which soil remediation must be considered.

The level 10 μg dl−1 is the lowest blood lead level that is considered elevated by CDC. There is nothing to be gained by using T < 10 μg dl−1. “No effective clinical or public health interventions have been identified that reliably and consistently lower BLLs that already are <10 μg dl−1. Establishing a level of concern substantially <10 μg dl−1probably would be accompanied by a sharp increase in misclassification of children as having an elevated BLL. The uncertainty associated with laboratory testing is too great to ensure that a single blood lead test reliably classifies individual children at levels <10 μg dl−1. This misclassification could confuse both parents and clinicians and expenditure of resources on testing that does not aid decision making“ (CDC 2002).

The slope or response of the blood lead–soil (dust) lead relationship (δ) has the units of μg Pb dl−1 blood increase per 1 mg kg−1 increment of soil or dust. SGV10 uses the default value of δ = 5 which is the arithmetic mean of studies reviewed by Duggan and Inskip (1985). It should be noted that the arithmetic mean is rarely a robust indicator of central tendency for environmental data from contaminated environments. Either the median or geometric mean are now widely used. The geometric mean of the Duggan and Inskip (1985) data is 4.1.

The SEGH task force regarded δ = 5 as an upper limit and ordinarily a lower value would be applied. Research over recent years has continued to support a general value of δ = 2.5 for residential and urban environments.

Alonso et al. (2001) determined the degree of lead and cadmium exposure in a population that resided in an area with contaminated soil (the Asua Valley on the outskirts of the Greater. Bilbao area in northern Spain); there are numerous industries in the valley. Soil pollution around homes was the main variable responsible for the difference in lead exposure levels among participants. Results indicated that a 200–1,000 μg g−1increase in soil lead would produce a 2.6 μg dl−1 increase in blood lead level or δ = 3.3.

von Lindern et al. (2003) reported data for the Bunker Hill Superfund site (Idaho, USA). Blood-to-soil (dust) slope factors were age dependent: for all age groups the slope factor was 0.9 μg Pb dl−1 per 1,000 μg g−1 house dust lead. The blood-to-soil slope was 1.5–2.5 μg Pb dl−1 per 1,000 μg g− 1 soil lead.

In Midvale, Utah (USA), Lanphear et al. (2003) evaluated the effect of soil abatement on children’s blood lead concentrations and on environmental levels of lead. The estimated reduction in blood lead for children of ages 6–36 months was 3.5 μg Pb dl−1 for every 1,000 μg g− 1 reduction in soil lead concentration.

Fewer newer studies have been published for urban areas since the recent emphasis in the USA has been on researching the effects of leaded paint removal in homes. Jin et al. (1997) conducted a comprehensive review of literature reports. Their cross-sectional studies yielded slope factors (δ) ranging from 1.1 to 2.25. These later studies for US industrial, mining and Superfund sites show that the value for delta for highly contaminated sites ranges from 0.9 to 3.5 with a mean of 2.5.

In SGV10, it is noted that “higher values of δ tend to be found in groups with: (1) children likely to exhibit peak exposure between 18 and 24 months; (2) dusty conditions, sparse vegetative cover; (3) homes with poor cleanliness and infrequent hand washing; (4) soil lead sources with slightly soluble lead salts such as that derived from automotive or stack emissions or well oxidised and more soluble sources including exterior paint; or (5) light textured or low organic matter soils”. These are more extreme conditions than are generally encountered. In paragraph 3.9 of SGV10 it is stated: “It is strongly recommended that the δ value is not changed without a detailed consideration and investigation of exposure patterns, the source of lead emissions and its relative bioavailability, and where possible measured concentrations of blood lead”. The recommendation appears unnecessarily stringent as it makes the worse case scenario the norm whereas one would expect it to be the exception.

SGV10 has a default of 1.4 for G, the Geometric standard deviation of the blood lead distribution. This is within the range suggested by SEGH and there is no reason not to concur with the SGV10 choice for the default value.

The default value for background blood value (B) in SGV10 is 3.44 μg Pb dl−1 It is noted in paragraph 2.24 of SGV10 that “the background blood lead concentration used in the derivation of the Soil Guideline Values includes a component attributable to soil and soil-derived indoor dust, and that the Soil Guideline Values overestimate the contribution from soil sources through this double-counting”. No recent surveillance data appear to have been published for blood lead in the UK but the experience in the USA is that levels have generally declined over recent years (Table 1). There is an argument therefore for entering B as 2.2 μg Pb dl−1 Table 1, when the calculated S is 2,340 mg kg−1, but in the absence of modern UK surveillance data the value B = 3.44 is probably best retained.

Variable n is the number of standard deviations corresponding to the degree of protection required for the population at risk. In deriving Soil Guideline Values an objective of 95% was set and n = 1.645. “That is, there is a 95% likelihood that an adult or child exposed to lead in soil at the Soil Guideline Value (taking into account the source and pattern of exposure implicit in each model) will have a blood lead concentration less than the health criteria value. Because of the conservative assumptions explicit in the SEGH models, and the degree of double-counting in the choice of baseline blood lead concentrations, this does not mean that the Soil Guideline Value is only protective of 95% of the general UK population” (SGV10). There seems no reason to amend the value of n.

Adult health

DEFRA document SGV10 also considers the problem of protecting adult health and provides a formula (equation 2.2 in SGV10) which adapts the SEGH model

$$ S = 1000\left[ {\frac{{(T/G^n ) - B}}{\delta }} \right] $$

by deleting δ and replacing the multiplicand 1,000 by [AT/(BKSF × IR × AF × ED)] where

  • AT is an averaging time = 15,695 days.

  • BKSF is a biokinetic slope factor relating (quasi-steady-state) increase in typical blood lead concentration to average daily lead uptake (0.4 μg dl−1 blood lead increase per μg day−1 lead uptake).

  • IR is the daily adult soil ingestion rate including outdoor soil and indoor soil-derived dust (0.04 g day−1): it is based on a default exposure model for the commercial/industrial land-use.

  • AF is the absolute gastrointestinal absorption factor for ingested lead in soil and lead in dust derived from soil (0.12 and dimensionless), based on an absorption factor for soluble lead of 0.2 and a relative bioavailability of 0.6 (soil/soluble).

  • EF is the exposure frequency for contact with contaminated soils and/or dust (230 days year−1).

  • ED is the exposure duration (43 years).

First, the equation printed in SGV10 appears to be incomplete. As printed, the equation, using the above DEFRA default values for the variables, yields a soil guidance value of 1,909 mg Pb kg−1 soil. However, if δ is reintroduced to comform with the original SEGH model and then set at δ = 2.5, the soil guidance value becomes 764 mg Pb kg−1 soil, close to DEFRA SGV10 guideline of 750 mg Pb kg−1 soil.

It is beyond the scope of this paper to analyse fully the assumptions used in the DEFRA computation for adults but two of the assumptions merit consideration. The parameter ED (exposure duration) is set at 43 years. This seems unrealistic as it appears to be the exposure for a maximum working life in full time employment in the same job and same environment. Lindsay (2003) reported that in 1989 the average person in the UK worked for 1,489 h each year and the average weekly hours of a manual worker was 43.5 in 1987; this equates to 34 weeks and a working day of 8.7 h. These reported data reduce AT to 7,310 days for one of the occupations most likely to involve direct exposure to the external environment. Over a 5-year period two-thirds of all people in the British workforce change jobs, and one in five makes two or more changes. Even among people in their 60s, one-third move to a new employer (McNair et al. 2004). A working life spent in only one occupation is no longer to be assumed.

It is acknowledged that since the 2002 publication DEFRA has undertaken further revision and now relies on its Contaminated Land Assessment (CLEA) model (DEFRA CLR 10 2002). More recently (DEFRA 2006) has indicated that revisions and refinements are now under consideration. It is considering the possibility of changing the Soil Guideline Value for lead from 450 to 1,200 mg kg−1 (Table C3 in DEFRA 2006). Nonetheless, SGV10 has not been withdrawn at the time of writing.

Societal implications of applying the DEFRA soil value

One newspaper report will suffice to illustrate the adverse societal impacts of a strict compliance with current DEFRA soil values.

The British national newspaper Sunday Telegraph carried a news item on 30 May 2004 concerning a house in Long Melford, Suffolk. The current resident and her two sisters inherited the house from their mother. To keep the house in the family, they wanted to arrange for two of the legatees to be bought out in order that the one daughter could continue to live there with her children. Their solicitors (lawyers) advised that, under new regulations, they could not apply for a mortgage without a certificate showing that they had carried out a full “environmental audit” including an assay of soil lead. Under new rules, they were told, they would have to have the garden tested for contamination. These tests revealed high soil lead and arsenic and the local council, Babergh, confirmed that, during the 19th century, the garden had, like hundreds of others in the area, been used for light industry; in this instance, a blacksmith’s forge and a hair processing factory. To obtain the certificate, it would be necessary to remove all the garden’s topsoil (500 tons) including trees and most of its established plants, at a cost of £41,000 (some US$80,000). Information is not available for the actual soil test value for the Suffolk house but the BBC regional television report “Look East” covered the issue and aired an interview with a local council representative. He stated on camera that lead in the soil exceeded the SGV10 soil Pb concentration of 450 mg kg−1 and the council was therefore obliged by law to make its recommendation for extensive remediation.

Many garden soils contain elevated lead concentrations. Culbard et al. (1988) reported a survey of garden soils in Great Britain. For 4,126 gardens, the geometric mean was 266 and range 13–14,100 mg Pb kg−1, respectively. For London boroughs (578 samples), the geometric mean was 578, range 60–13,700 mg Pb kg−1, respectively. London is made up of 33 boroughs and more than 7 million people live in London with more than 3 million households, making it the biggest city in Western Europe. London contains 15% of England’s total population. It can be inferred from the reported London soil data that some 1.5 million households are characterised by gardens with a soil Pb content above SGV10 concentration and removing the soil could therefore amount to as much as £60 billion overall.

In practice, individual authorities have not necessarily followed DEFRA guidance levels. The Derbyshire Peak District was intensively mined for lead in earlier centuries and soil contamination is widespread (Cotter-Howells and Thornton 1991). Derbyshire Dales District Council (DD 2006) has issued a guidance note for land development when Pb values fall within the target area. The authority used a delta value of 2 yielding a residential Soil Guideline Value for children of S = 1,155 mg Pb kg−1. “To reflect the fact that the majority of soil lead contamination in the district is naturally occurring, or has arisen from mining contamination, and that gardens typically have well-maintained vegetative cover, soil lead concentrations (mean value test) less than 1,155 mg/kg will not be required to be remediated for new developments” (DD 2006).

Conclusions

To protect child health, SGV10 proposes a soil Pb concentration of 450 mg kg−1, a rounding down from the computed value of 462 mg kg−1. The above discussion has shown that this soil guidance value arises from choosing extreme values for variables B and δ. If values are chosen which more fairly reflect CDC BLL advice, and if protection is to be afforded to the typical child in a typical environment rather than the extreme “dusty conditions, sparse vegetative cover or homes with poor cleanliness and infrequent hand washing”, then calculated S values are severally 922 mg Pb kg−1 (T = 14, δ = 5); 924 mg Pb kg−1 (T = 10 and δ = 2.5); and 1,844 mg Pb kg−1 (T = 14 and δ = 2.5).

It is not the purpose of this paper to argue for any particular soil advisory content, only to demonstrate the sensitivity of the SEGH model to the choice of variable values. Soil lead content is an important predictor of children’s risk for an elevated BLL. Soil samples taken from play areas in a yard (garden) have a stronger relationship to children’s BLLs than samples from other locations. The US EPA defines a soil lead hazard as bare soil that contains 400 mg kg−1 of lead in a play area or 1,200 mg kg−1 in other parts of a garden (Federal Register 2001).

The choice of guidance values for industrial and commercial environments is especially problematic and a ‘one size fits all’ lead content is self-evidently inappropriate. Some individuals may spend their working days high in office buildings with windows that do not open and breathe only filtered and conditioned air; the lead content of any adjacent shrubbery beds is probably irrelevant. Others (e.g., operators of excavating equipment in brownfield sites or gardeners in public amenity areas) experience a very different environment and the presence of toxic substances in the soil has a high significance for their health. A strong case can be made for separating worker health from child health and examining the former issue in greater depth.

Anecdotal evidence suggests that when guidance documents pass into the hands of officials who have neither the scientific nor legal training to interpret them, serious societal difficulties arise. DEFRA have now issued clarification of the legal status of SGV10 (DEFRA 2005). British statutory guidance (which deals with the manner in which the determination of contaminated land is to be carried out) make it clear that to determine land as contaminated land on the grounds of a “significant possibility of significant harm” to human health, the local authority must be satisfied that: “the amount of the pollutant in the pollutant linkage in question: which a human receptor in that linkage might take in, or to which such a human might otherwise be exposed, as a result of the pathway in that linkage, would represent an unacceptable intake or direct bodily contact, assessed on the basis of relevant information on the toxicological properties of that pollutant”. It should be a matter for careful consideration by local authorities whether concentrations of substances in soil equal to, or not significantly greater than, an SGV would meet the legal test. Soil Guideline Values would not necessarily satisfy that legal test.

The problem of individual interpretation of Soil Guideline Values has now been recognised by DEFRA: “a range of multidisciplinary skills is needed to understand and carry out risk assessment both within regulatory bodies and in the external organisations dealing with land contamination. Technical guidance must be clear about what is required and what level of understanding is needed, so that appropriate skills can be applied “(DEFRA 2006).

The societal costs of over-strict environmental legislation can be severe. To avoid this the relevant scientific community should be perceived as stakeholders in the legislative process and its outcome. Those whose expertise provided the original research data need to be consulted when legislation is drafted. Additionally, the scientific community should accept the responsibility of subsequent public comment on national environmental legislation and guidelines and not remain silent and apparently disconnected.