Air Change Rate - Input Parameter Protocol

Room air change rate is an input parameter for the WHO HOMES and PT models and is the primary driver in the removal of pollutants generated in any given room. Air change rate is typically measured by analyzing how quickly tracer gas concentrations decrease in a room after the source of those gas emissions has been stopped or removed. The faster the decay rate of the gas concentration, the higher the air change rate, and vice versa.

This protocol describes the process to measure decay rates and calculate the estimated air change rates from those data. It is meant to be applied to a room (or kitchen) with four walls and a ceiling, as defined in the kitchen volume protocol. A description of how to perform post-hoc analysis to derive air change rates from previously collected air quality data is provided in Annex A.

The tracer gas method used in this work relies on the rapid cessation of emissions from the given source, so that the decay of tracer gas concentrations in the room of interest may be measured without contamination from any additional tracer gas introduced during the decay period. The best source of tracer gas is thus one that can be removed quickly from the room in which the air change rate is being assessed.

There are many viable tracer gas source candidates, including CO and CO2. Ideally, any tracer gas should be easy for a single person to move and operate safely, and large enough to substantially increase the tracer gas concentration levels in the room of interest on the time scale of minutes (e.g., a candle will likely not produce sufficient CO or CO2 to be used in most rooms).

Note that special care should be taken if using CO to minimize exposure, as CO can have acute health impacts at high concentrations. Specifically, the concentrations should be monitored and steps taken to ensure the minimum durations and magnitude of exposures occur. A stove or gas canister can serve as the source of the tracer gas. If a stove is being used as the source of the tracer gas, a stove that can be quickly and safely moved to a downwind location outside the room, such as a charcoal stove, is better suited than a stove like a three-stone fire or chimney stove which cannot be removed easily.

A single tracer gas emission may last only 1-10 minutes and the corresponding concentration decay should be measured for at least 20 minutes. The complete air change rate measurement protocol may take 1-3 hours to complete in a household, due to equipment set up, number of tracer gas concentration decay events measured, and interaction with the household. During the measurement, only a single tracer gas source (e.g., a single stove or a single gas canister) will be used. Thus, the household should be notified that before beginning the test, no other combustion sources can be used during the duration of the test, which could include other stoves, heaters, incense, lamps, candles, mosquito coils, or burning of trash. If CO2 is being used, humans should leave the room for a period of time before the test to prevent “contamination” from human-generated CO2.

The air change rate calculated from the measured data will be representative of the ambient wind speed and temperature conditions at the time of measurement, so performing the measurements in a variety of conditions is advisable.