Particle settling after lead-based paint abatement work and clearance waiting period

This study investigated the evolution of airborne particle concentration an d size distribution following abatement work in a controlled environment ut ilizing direct real-time particle monitoring and used it to project potenti al lead loadings as those particles settle. An 860 ft(3) environmental test chamber with sophisticated ventilation and air purifying systems was built . Wooden doors with lead-based paint were dry sanded or scraped to generate the highest feasible airborne lead concentrations. Size-fractional airborn e particle concentrations decreased exponentially with time in all tests, e ven with no air exchange, consistent with the stirred model of constantly m ixed air, which predicts longer settling than for tranquil settling. Very l ow levels of air mixing generated by temperature gradients and initial room air turbulence affected particle settling. About 90% of airborne lead mass settled within 1 hour after active abatement, before final cleaning began. During the second waiting period of 1 hour, which followed cleaning of the floor, additional dust settled so that the additional potential lead loadi ng from remaining airborne lead was less than 20 mug/ft(2). For this worst case scenario, the underestimate of the lead loading done by the clearance sampling did not exceed about 30%. For more realistic conditions, the under estimates are projected to be much lower than the new 40 mug/ft(2) Housing and Urban Development (HUD) clearance standards for floor dust lead. These results were obtained for the first waiting period (between the end of acti ve abatement and the beginning of cleaning) of 1 hour, as recommended by HU D guidelines. Thus, this study demonstrates no need to increase either the first or second waiting period.