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' 9 generally range from S - 50 parts per million'. Also, industrial sources such as smelters, recycling facilities, and _ mining activities can result in lead contamination at residential areas . This adds difficulty in relating lead levels in soil to potential health effects because lead from different sources may pose different levels of potential hazard. One apparent difference is the extent to which ingested lead originating from different ' sources is taken up into the body- -that is, the bioavailability of the lead. Decisionmakers should consider this and any other available information when implementing the recommendations contained in this guidance, particularly where non-paint sources of lead are involved. That is , if the soil is contaminated by lead from other sources, rather than lead-based paint , decisionmakers should investigate the types of lead compounds present and their unique characteristics . Agency guidance on consideration of bioavailability of lead in risk assessment can be found in the Guidance Manual for the Integrated Exposure Uptake Bickinetic Model for Lead in Children (available from National Technical Information Service, U. S . -Dept . of Commerce, Attn: Sales, Springfield, VA 22169 (703/487-4650) , as document number PB 93-963510) . S- il lead concentrations in the Unit :d States vary widely, from less than one to tens of thousands of parts per million (ppm) . This range of concentrations and attendant potential exposure levels indicates that it is appropriate to develop a scaled strategy of risk reduction activities , depending upon the concentrations at particular locations and other site-specific factor . The Agency' s recommendations for response activities at varying soil lead concentrations are as follows . The Agency is recommending that (depending upon use patterns , populations at risk, and other factors) , when lead concentrations are observed that exceed 400 ppm in bare soil , further evaluation should be undertaken and physical exposure- reduction activities , commensurate with the expected degree of risk, are appropriate .' The Agency believes that the 400 ppm 'U. S . Environmental Protection Agency (1989) Review of the National Ambient Air Quality Standards for Lead: Exposure Analysis Methodology and Validation. U.S. EPA Office of Air Quality Planning and Standards , RTP, NC. EPA-450/2-89/011 . 'The selection of 400 ppm in this guidance is based upon two decisions . The first is that the level should help in reducing the threat that environmental lead poses to the public . In this guidance, EPA estimates that beginning exposure reduction activity at 400 ppm will help ensure that a typical child or group of children exposed to lead would have an estimated risk of no more than 5% of exceeding a blood lead level of 10 pg/dl . This benchmark may change in the future section 403 rulemaking. 9