Respirable crystalline silica exposures during asphalt pavement milling at eleven highway construction sites

Asphalt pavement milling machines use a rotating cutter drum to remove the deteriorated road surface for recycling. The removal of the road surface has the potential to release respirable crystalline silica, to which workers can be exposed. This article describes an evaluation of respirable crystalline silica exposures to the operator and ground worker from two different half-lane and larger asphalt pavement millingmachines that had ventilation dust controls and water-sprays designed and installed by the manufacturers. Manufacturer A completed milling for 11 days at 4 highway construction sites inWisconsin, andManufacturer B completed milling for 10 days at 7 highway construction sites in Indiana. To evaluate the dust controls, full-shift personal breathing zone air samples were collected from an operator and groundworker during the course of normal employeework activities of asphalt pavement milling at 11 different sites. Forty-two personal breathing zone air samples were collected over 21 days (sampling on an operator and ground worker each day). All samples were below 50 mu g/m(3) for respirable crystalline silica, the National Institute for Occupational Safety and Health recommended exposure limit. The geometric mean personal breathing zone air sample was 6.2 mu g/m(3) for the operator and 6.1 mu g/m(3) for the groundworker for the Manufacturer Amillingmachine. The geometric mean personal breathing zone air samplewas 4.2 mu g/m(3) for the operator and 9.0 mu g/m(3) for the groundworker for theManufacturer B millingmachine. In addition, upper 95% confidence limits for themean exposure for each occupation werewell below50 mu g/m(3) for both studies. The silica content in the bulk asphalt material beingmilled ranged from7-23% silica for roadsmilled by Manufacturer A and from 5-12% silica for roadsmilled by Manufacturer B. The results indicate that engineering controls consisting of ventilation controls in combination with water-sprays are capable of controlling occupational exposures to respirable crystalline silica generated by asphalt pavement milling machines on highway construction sites.