Revisiting the effect of room size and general ventilation on the relationship between near- and far-field air concentrations.

Objectives: In 1999, Cherrie carried out a series of mathematical simulations to investigate dispersion of pollutants through two indoor zones: the near-field (NF) and the far-field (FF). The results of these simulations were used to derive modifying factors for use in exposure modeling. However, in the original simulations, no account was taken of deposition on surfaces, either from sedimentation of aerosols or other mechanisms or the potential effects of intermittent or short duration sources. These factors may affect pollutant dispersion, particularly the relationship between NF and FF levels. The Advanced REACH Tool (ART) is based on a two-zone dispersion paradigm. Further simulations have been carried out to help ensure that the ART realistically reflects pollutant dispersion.

Methods: Pollutant dispersion has been simulated using a two-compartment well-mixed box model to represent the NF and the FF. Simulations were repeated for a range of room sizes and ventilation conditions. Intermittent sources (e.g. batch processes) were simulated by having the source active for 1 h followed by a 1-h gap, while short duration work emissions were set to last for 10 min, 30 min, 1 h, or 4 h, within the working day. Deposition was modeled by adding an equivalent air exchange rate based on published research data. Simulations were undertaken for non-volatile, monodisperse aerosols of aerodynamic diameter: 0.3, 1, 3, 10, 30, and 100 μm and the results were then interpreted in terms of typical polydisperse industrial aerosols.

Results: Room size and general ventilation strongly influenced dispersion from the NF to the FF as Cherrie had originally found. When varying the duration of the simulation, the biggest difference from continuous work was seen in small poorly ventilated rooms, with the ratio of the NF to FF concentration for 1-h work in the smallest room and lowest air exchange rate being a fifth of that calculated for continuous work. For large rooms and high general ventilation rates, the duration of the activity made little difference to dispersion. The results suggest that for the purposes of dispersion intermittent batch work is equivalent to continuous work. For typical simulated poly-disperse aerosols, the main effect of aerosol deposition was to reduce the predicted high concentrations compared to vapours when working in confined spaces.

Conclusions: Both short duration of source emissions and deposition of aerosols have an important effect on dispersion, and the results from this study have been reflected in the ART model.