Elevated disease rates have been documented among construction workers for cancer, pneumonoconiosis, asbestosis, and silicosis. However, methodologies for exposure assessment in construction are not well described in the U.S. literature. Working through a cooperative agreement with the National Institute for Occupational Safety and Health (NIOSH), the Center to Protect Workers' Rights--a research arm of the Building and Construction Trades Department, AFL-CIO--has developed and used a "Task-Based Exposure Assessment Model (T-BEAM)" for construction. The characteristic elements of T-BEAM are: (1) an emphasis on the identification, implementation, and evaluation of engineering and work practice controls; and (2) use of experienced, specially trained construction workers (construction safety and health specialists) in the exposure assessment process. A task-based approach was used because tasks, or specialized skills, form the single greatest thread of continuity in the dynamic environment of construction. Workers in the construction industry come from several crafts and are typically employed by a large number of contractors throughout their career. Project types (e.g., residential or industrial rehabilitation) are also highly variable and present unique health risks. Finally, because construction involves building, renovating, or dismantling physical surroundings, the work site is constantly changing. Between 1995 and 1996, T-BEAM was applied to the collection of approximately 200 personal exposure measurements associated with "hot work tasks"--welding and thermal cutting. Data were collected with the assistance of specially trained, journeyman ironworkers, pipe fitters, and boilermakers on nine construction sites located throughout the United States. Portable local exhaust ventilation was provided to participating contractors with the intent of measuring its impact on exposure. Results indicate that data collected in a standardized, systematic fashion from multiple work sites can be used to characterize exposures among sampled trades. Comparison of results to American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values (TLVs) demonstrate a significant health hazard among sampled trades posed by welding and thermal cutting fume, manganese, nickel, and chromium VI. Direct estimates of the probability of exceeding the ACGIH TLV for respirable particulate suggests that boilermakers (100%) and ironworkers (71%) are at greatest risk. Other task variables evaluated with respect to exposure include task, whether work was performed indoors or outdoors, intermittency of work, and use of ventilation. Use of local or mechanical ventilation reduced mean exposures to fumes significantly.