Children are at most risk because of their still developing brains and bodies. Because daily contact with 80 ppm lead in soil can raise a child’s blood lead level by one unit, 80 ppm is the precautionary standard in risk assessment (i.e., soils with lead levels greater than 80 ppm should not be used for gardening unless remediated).
DPH recommends the following best practices for reducing lead exposure from gardening: practice good personal hygiene (wear gloves while gardening, wash hands frequently, wash produce well before consuming), garden on low-leaded soil (< 80 ppm) when possible, and prevent further soil contamination by looking for surrounding risks. Other best practices include amending soil with organic matter (clean compost) and maintaining neutral pH (add limestone if soil is too acidic). More information at http://www.sfdph.org/dph/files/EHSdocs/ehsCEHPdocs/LeadHazardUrbanGardening.pdf.