Controlling Active Ventilation to Safeguard the Community from Indoor Releases of Acutely Toxic Chemicals

摘要

U.S. EPA's Risk Management Program (RMP) requires the evaluation of worst-case and more realistic alternative release scenarios. While these are useful constructs for emergency planning, the appropriate real-time response required to mitigate the potential for harmful off-site exposure often requires a more detailed assessment. A case in point is where there is the potential for an acutely hazardous vapor or gas to be emitted from an indoor process that is released into a mechanically ventilated space. Under normal operations, the forced ventilation would exhaust the released gas to the atmosphere after passing through a pollution control system. When an upset condition causes a release to occur that bypasses the control system, as a safeguard to on-site workers and the public, the ventilation fan is turned off through an emergency procedure or an interlock. High concentrations of the substance are trapped in the room which will passively ventilate to the atmosphere. EPA's RMP Off-site Consequence Modeling Guidance suggests a reduced modeled release rate to the atmosphere to account for such an indoor non-ventilated release. While this may result in suitable mitigation immediately after the release, it does not address the potential adverse effect on public receptors when the forced ventilation of the indoor area housing the process is resumed. As such, site-specific guidance is required for the facility operators who will seek to exhaust and replenish the indoor air as soon after the release has ceased as is possible and still safeguard the public. To address this situation a spreadsheet-based computer program has been developed that links a single-compartment indoor air quality model with an ambient dispersion model such as AERMOD if the release to the outdoors is positively or neutrally buoyant, or a suitable heavy-gas model if the vented release is negatively buoyant. Based on a specified indoor release of finite duration, the spreadsheet computes an exhaust fan reactivation time that ensures that the exhausted release will not result in off-site concentrations that exceed a specified emergency response exposure level such as an Acute Exposure Guideline Level (AEGL), Emergency Response Planning Guideline or the RMP toxic endpoint. The method can be tailored to real-time estimates of basic meteorological factors such as wind speed category and wind direction sector. The program can also access and plot on a base map the distance to various ranges of potential effects (e.g., AEGL 1, 2. or 3) associated with variations in release rate, duration and timing of the ventilation. This paper will describe the situations to which the methodology is applicable, provide a detailed description of the technical approach and provide an example application. The suitability of this type of approach for emergency preparedness and response planning will also be discussed.