Carbon Monoxide and Oxides of Nitrogen Exposure during Offloading of Car Carrier Vessels

Abstract

A field study was conducted from April to October 1994 to evaluate worker exposure to carbon monoxide (CO) and oxides of nitrogen (NO_x) during offloading of vehicles from car carrier vessels. Personal and area samples for CO and NO_x were taken during offloading operations aboard ten ships dedicated to vehicle cargo. Personal exposure results can be summarized as follows: 20 diffusion tube samples in the breathing zones of merchant seamen monitoring offload operations gave time-weighted average (TWA) CO concentrations of 17 ppm (geometric mean) with a range of 2 to 52 ppm. Similar TWA measurements for nitrogen dioxide gave a geometric mean of <1 ppm. Ventilation measurements (air changes per hour) based on exhaust air flow rate were from 0.4 to 32.7, and based on supply air flow were 12.5 to 101.5. Offloading rates ranged from 89 to 394 vehicles per hour. There were 5 to 12 vehicles being operated at a given time. The time required to drive a vehicle off the deck ranged from 16 to 71 seconds. Twenty-five to 90 workers were exposed per offload, primarily stevedores and merchant seamen. No correlation was found indicating that vessels with a greater air change rate had lower pollutant concentrations on the individual car decks. Nine of the ten ships sampled exceeded the regulatory standards set forth in the U.S. Code of Federal Regulations (49 CFR 176.79), four ships exceeded the Occupational Safety and Health Administration ceiling standards for CO, and 45 percent of the merchant mariners monitored exceeded the American Conference of Governmental Industrial Hygienists threshold limit value (8-hour TWA) for CO. Recommendations to reduce exposure include monitoring pollutants and ventilation during offload operations and implementing work practice controls if concentrations are elevated; use of breakout drivers to minimize vehicle operating time on deck; and removal of padeyes to prevent securing of vehicles adjacent to ventilation ducts.