Guide for Radon Measurements in Public Buildings

2. Radon Measurement Duration

2.1 Long-Term Measurements

Radon levels in a home or other building can vary significantly over time. In fact, it is not uncommon to see radon levels change by a factor of 2 to 3 over a 1-day period and variations from season to season can be even larger. Higher radon levels are usually observed during winter months. As a result, a long-term measurement period will give a much better indication of the annual average radon concentration than measurements of shorter duration. Long-term measurements are typically 3 to 12 months in duration. During this type of measurement, there are no requirements for the occupants to change their life-style once the measurement devices have been put in place. Health Canada recommends that the radon test performed in a home or public building be a long-term measurement. Health Canada does not recommend a test of duration less than 1 month, a minimum of 3 months is recommended and 12 months is optimum.

2.2 Short-Term Measurements

In rare cases, a more rapid indication of the radon concentration may be required. Under such circumstances a short-term measurement of duration less than 3 months (more typically 2 to 7 days) can be performed. However, short-term measurements should be used with caution for the reasons cited above. Testing durations of less than 2 days (48 hours) are never acceptable to determine radon concentrations for purposes of assessing the need for remedial actions. Since radon concentrations vary over time, it is strongly recommended that the result of any short-term measurement be confirmed with a "follow-up" long-term measurement. The follow-up measurement should be made at the same location as the initial measurement. A single short-term measurement is not a sufficient basis for a decision to mitigate. In this case a follow-up measurement is always necessary for mitigation decision-making regardless of the initial measurement result.

2.2.1 Conditions for Short-Term Measurements

Short-term measurements must be made under closed-building conditions to stabilize the radon concentrations and increase the validity of the annual radon concentration estimate. In addition to maintaining closed-building conditions during the measurement, these conditions should be in place for 12 hours prior to the initiation of a measurement lasting less than 4 days, and are recommended prior to measurements lasting up to a week in duration. Closed-building conditions involve ensuring that:

• Windows on all levels and external doors are kept closed for the duration of the test, except during normal entry and exit. Normal entry and exit include a brief opening and closing of a door, but external doors should not be left open for more than a few minutes.
• External-internal air exchange systems such as high-volume, whole-house and window fans are not operated. However, attic fans intended to control attic temperature or humidity may be operated. Combustion or furnace makeup air supplies must not be closed.
• Normal operation of permanently installed energy recovery ventilators (also known as heat recovery ventilators or air-to-air heat exchangers) may continue. In houses where permanent radon mitigation systems have been installed, these systems should be functioning during the measurement period.
• Air conditioning systems that recycle interior air can be operated during the closed-building conditions.

Short-term measurements lasting less than four days should not be conducted during severe storms or periods of unusually high winds. The rapid changes in barometric pressure associated with storms increase the chance of a large difference in the building interior and exterior air pressures, thus changing the rate of radon influx. A high wind increases the variability of radon concentration because of wind-induced differences in air pressure between the building interior and exterior. In either case, the radon concentration during the measurement may not be representative of the average concentration in the building. Weather predictions available on local news stations or weather-reporting Web sites provide sufficient information to determine if these conditions are likely.

Closed-building conditions generally prevail during the cold season from October to April when the average daily temperature is low enough that windows are kept closed. To provide closed-building conditions outside the cold season, the occupants may have to change their life-style for the duration of the measurement.