A Tool to Predict Exposure to Hazardous Air Pollutants
The Clean Air Act Amendments of 1990 authorized the regulation of 189 hazardous air pollutants (HAPs) that cause cancer, reproductive harm, or other serious health problems. Current regulations set source-specific limits on emissions, but the U.S. Environmental Protection Agency (EPA) plans to develop future rules that focus on reducing human exposure to these compounds. To develop these rules, it will be necessary to predict exposures. Even though most HAPs are emitted by outdoor sources such as vehicles and industrial facilities, Americans spend about 90 percent of their time indoors, so most human exposure to these pollutants takes place after they have entered buildings. Predicting exposure thus requires an understanding of the processes that can affect pollutants indoors. Toward this goal, researchers in the Atmospheric Sciences and Indoor Environment Departments of the Environmental Energy Technologies Division (EETD) at Lawrence Berkeley National Laboratory (Berkeley Lab) are developing a computer-based modeling tool that simulates the key processes, including ventilation, chemical reactions between gases, and sorption of pollutants on material surfaces.
In this context, sorption is the reversible attachment of gas molecules to indoor materials. Adsorption describes the process by which the gas molecules stick to surfaces. Desorption is the reverse process, in which molecules that were previously sorbed to a surface are reemitted into the air. In a sealed room, these two processes will eventually reach equilibrium, with a fraction of the pollutant remaining in the air and the rest sorbed to surfaces.
Experiments to Study Sorption of Pollutants Indoors
To acquire a detailed understanding of sorption in realistic settings, Berkeley Lab researchers conducted experiments in a room-sized test chamber constructed and furnished to simulate a residential environment. The room is finished with painted gypsum wallboard and padded carpet and furnished with wood and veneer tables, desks, and bookcases; upholstered chairs; and cotton draperies. Twenty air pollutants, including many HAPs and key components of environmental tobacco smoke, were released into the chamber, and their concentrations were monitored over time.
The typical observed pattern is shown in Figure 1 for xylene, a compound that exhibited a moderate amount of sorption. With the room initially sealed, the observed decay in gasphase concentrations reflected adsorption of compounds to material surfaces. After several hours of adsorption, concentrations stabilized, indicating that equilibrium had been reached. The room was then ventilated (flushed) at a very high rate to quickly remove all gaseous pollutants. Rising concentrations after the room was resealed at hour 25 resulted from desorption of the previously sorbed mass. Time-concentration patterns for all 20 compounds were fitted to mathematical equations to determine the simplest model that could explain the observed behavior of each compound. Specifically, the goal was to understand the rates of adsorption and desorption and the overall sorption tendency (i.e., equilibrium) for each compound in the furnished room.
Indoor Sorption Sorption Alters Pollutant Exposure
Experimental and modeling results indicate that many important HAPs adsorb to surfaces at rates equal to or faster than ventilation rates in typical homes. This means that once a pollutant enters a residence along with outside air, a substantial fraction of the pollutant may stick to surfaces before it can be removed with air leaving the building. When outdoor pollutant concentrations are high, sorption will reduce the concentrations encountered indoors. However, desorption later on will means that indoor levels will be higher than those outdoors for a time as some pollutant returns to the indoor atmosphere. The consequence is a difference in the temporal pattern of indoor versus outdoor concentrations, which has important implications for human exposure. Figure 2 shows the results of a simulation using our sorption model with a repeating outdoor concentration profile. The indoor concentration pattern for toluene, a compound that does not sorb readily, is similar to the outdoor profile but lags behind the outdoor concentrations because of the time it takes for air to enter buildings from outdoors. The pattern is markedly different for the highly sorbing pollutant cresol. The implication is that indoor cresol exposures will be approximately constant throughout the day and not depend significantly on the outside concentrations.
Both the rates of adsorption and the potential extent of sorption at equilibrium varied widely among the pollutants studied. The most rapidly sorbing compounds tested were gas-phase polycyclic aromatic hydrocarbons (PAH), cresols, and the tobacco smoke constituent nicotine. These compounds sorbed much faster than ventilation air exchange rates and were more than 95 percent sorbed at equilibrium (i.e., less than five percent remained in the air). Two of the HAPs that have generated the most concern to date, benzene and acrolein, were observed to adsorb at relatively slow rates, suggesting that indoor exposure patterns for these compounds may not be greatly affected by sorption.
For more information, contact:
- Brett Singer
- (510) 486-4779; fax (510) 486-5928
This research was funded by the Department of Energy through the National Petroleum Technology Office and the Western States Petroleum Association. Also contributing to this research are Nancy Brown (principal investigator), Alfred Hodgson, Toshifumi Hotchi, and Kenneth Revzan.