A Comparison of Laboratory Results to Field XRF Results for Lead in Paint Chips
Since the introduction of the EPA/HUD Guidelines, pursuant to the Residential Lead-Based Paint Hazard Reduction Act (Title X) in 1992, there has long been a misunderstanding about inconsistencies between laboratory results and field XRF results for lead in paint chips. This brief is intended to address those discrepancies by providing and explanation of how and why the two means of reporting are so often different. From Appendix 1 of the EPA/HUD Guidelines:
One cannot convert from ppm or % by weight to area concentration (mg/cm2 ) as measured by an X-ray fluorescence instrument in any predictable way unless the total mass per unit area of the sample is known. One reason is that the dilution factor of adding more non-leaded paint layers over an existing leaded one will not change the area concentration. However, adding additional layers of paint will change the % by weight. The area concentration is independent of the thickness of the paint layers. The XRF determines the lead mass per unit area as measured by X-ray emission from a lead layer (mg/cm2 ). The weight percent method measures the percent of lead in the bulk paint films by determining the weight of lead in the total paint sample.
XRF technology has evolved considerably from the early days of the Guidelines but even today the common field XRF units still utilize technology which measures lead density (mg/cm2 ) while quantitative laboratory analysis provides lead concentration (% by weight). Only under very specific conditions can those two values provide a comparable risk assessment conclusion:
- if the paint chip sample was collected for the entirety of all paint over a known measured area, and
- if the sample size was small enough that the laboratory could digest the entirety of the sample or if not it has a density of specifically 0.20 grams per square centimeter and can be thoroughly homogenized, and
- if the entirety of the sample could be completely isolated from the sample container used for collection
Samples meeting all of the conditions above are infrequent through no fault of the laboratory. Yet it is often challenging for the sample collector in the field to meet those conditions. And when any one of the conditions is not met the regulatory metrics of 1.0 mg/cm2 and 0.5% by weight can and usually will diverge considerably. Further exacerbating the discrepancies is that when lab results in % weight are artificially converted to units of mg/cm2 there is a high probability that the laboratory result will be lower than the field XRF result.
Summary: Expecting laboratory paint chip lead results to routinely align with field XRF lead results is possible but not realistic, thus asking a lab to report their % wt. results in mg/cm2 units is usually counterintuitive and misleading. A report comment should be applied in any case where the laboratory reports lead paint chip results in units of mg/cm2 .
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