Chromate concentration bias in primer paint particles.

Chromate-containing primer paints are used to prevent corrosion on metal surfaces. Chromate contains hexavalent chromium (Cr6+), a human carcinogen. The objective of this research was to determine if there is a bias in the fraction of chromate found in various particle sizes generated during primer painting operations. A solvent-based, aviation primer paint was sprayed using a high-volume, low-pressure spray gun. Paint particles were collected and separated by size with seven-stage cascade impactors. It was determined that particles with a mass aerodynamic diameter < 2.0 microm contained significantly less Cr6+ per dry weight of paint than particles > 2.0 microm (P < 0.001). The median concentration of Cr6+ in particles < 2.0 microm is 18 micro g of Cr/mg of dry paint and the median concentration for particles > 2.0 microm is 70 microg of Cr/mg of dry paint. The mixed paint contains 18.75% strontium chromate, which equates to a ratio of 67 microg of Cr/mg of dry paint. Particles > 2.0 microm are more likely to impact in the upper tracheobronchial regions of the lung where mucociliary clearance is relatively rapid. Additionally, chromate emissions from spraying operations may be overestimated because larger particles, which are more easily trapped on an air filter, contain more chromate than the smaller particles, which are more likely to bypass an air filter.

Validation for a recirculation model.

Recent Clean Air Act regulations designed to reduce volatile organic compound (VOC) emissions have placed new restrictions on painting operations. Treating large volumes of air which contain dilute quantities of VOCs can be expensive. Recirculating some fraction of the air allows an operator to comply with environmental regulations at reduced cost. However, there is a potential impact on employee safety because indoor pollutants will inevitably increase when air is recirculated. A computer model was developed, written in Microsoft Excel 97, to predict compliance costs and indoor air concentration changes with respect to changes in the level of recirculation for a given facility. The model predicts indoor air concentrations based on product usage and mass balance equations. This article validates the recirculation model using data collected from a C-130 aircraft painting facility at Hill Air Force Base, Utah. Air sampling data and air control cost quotes from vendors were collected for the Hill AFB painting facility and compared to the model's predictions. The model's predictions for strontium chromate and isocyanate air concentrations were generally between the maximum and minimum air sampling points with a tendency to predict near the maximum sampling points. The model's capital cost predictions for a thermal VOC control device ranged from a 14 percent underestimate to a 50 percent overestimate of the average cost quotes. A sensitivity analysis of the variables is also included. The model is demonstrated to be a good evaluation tool in understanding the impact of recirculation.

The impact of recirculating industrial air on aircraft painting operations.

The 1990 Clean Air Act Amendments resulted in new environmental regulations for hazardous air pollutants. Industries such as painting facilities may have to treat large volumes of air, which increases the cost of an air control system. Recirculating a portion of the air back into the facility is an option to reduce the amount of air to be treated. The authors of this study developed a computer model written in Microsoft Excel 97 to analyze the impact of recirculation on worker safety and compliance costs. The model has a chemical database with over 1300 chemicals. The model will predict indoor air concentrations using mass balance calculations and results are compared to occupational exposure limits. A case study is performed on a C-130 aircraft painting facility at Hill Air Force Base, Utah. The model predicts strontium chromate concentrations found in primer paints will reach 1000 times the exposure limit. Strontium chromate and other solid particulates are nearly unaffected by recirculation because the air is filtered during recirculation. The next highest chemical, hexamethylene diisocyanate, increases from 2.6 to 10.5 times the exposure limit at 0 percent and 75 percent recirculation, respectively. Due to the level of respiratory protection required for the strontium chromate, workers are well protected from the modest increases in concentrations caused by recirculating 75 percent of the air. The initial cost of an air control system is $4.5 million with no recirculation and $1.8 million at 75 percent recirculation. The model is an excellent tool to evaluate air control options with a focus on worker safety. In the case study, the model highlights strontium chromate primers as good candidates for substitution. The model shows that recirculating 75 percent of the air at the Hill painting facility has a negligible impact on safety and could save $2.7 million on the initial expenses of a thermal treatment system.