Radon Measurement Protocols

Objective and contents of radon measurement protocols

A–2.

The results of radon measurement are used to inform both employers and workers on the level of exposure of workers (and hence the associated health risks) and the need to optimize protection to keep exposures as low as reasonably achievable.

A–3.

The objective of radon measurement is to assess the activity concentration of radon and, where appropriate, its progeny. The measurement protocol may include the following:

Measurement systems for radon in workplaces

A–4.

The detectors for radon are usually not sensitive to thoron. Passive detectors such as solid state nuclear track detectors are commonly used for long term radon measurements in workplaces. The measurements provide results over a specific period. Continuous radon monitoring can also be used for the measurement of radon activity concentration in workplaces. This type of monitor provides a time series of the radon activity concentration over the measurement period. Continuous radon monitors identify the variation of radon activity concentration (e.g. daily, seasonal, activities, occupancy periods) so as to provide information to better assess the exposure of workers.

Calibration and quality management for radon measurements

A–5.

Continuous radon monitors are usually calibrated individually by an authorized organization, which could be the manufacturer or a service agent. Equipment used for radon measurements needs to be calibrated at regular intervals, as recommended by the manufacturer (typically annually), and following any repair or certain types of maintenance. Following calibration, a certificate needs to be provided, which includes the calibration date, results of the test and the traceability to international or national standards.

A–6.

Passive devices are calibrated in batches under different conditions, such as a controlled radon concentration and at a specified temperature and humidity. In certain cases, calibration at different activity concentrations can be performed. Recommendations on service providers are provided in Appendix II.

Duration of radon measurements

A–7.

Measurements need to be capable of determining the annual average activity concentration of ²²²Rn in the workplace (e.g. taking into account the measurement season and duration of exposure). The most reliable way is to take measurements for a whole year. However, shorter sampling durations can be used if they are considered capable of providing results that are representative of the annual average radon level. This can be achieved by placing detectors for an entire year, or else using a series of detectors placed for periods of a few months to cover the whole year.

Seasonal correction factor for radon measurements

A–8.

In some countries, seasonal correction factors are applied to measurement results so that the results of measurements taken over part of a year can be extrapolated to provide an annual average. Seasonal correction factors are dependent on local factors such as climate, the type of building and the characteristics of the workplace. They are not applicable to other situations unless they have been evaluated as also being appropriate for wider use.

A–9.

Reference [A–1] provides methodologies for radon measurements, including long term measurements with a minimum duration of two months. According to Ref. [A–1], at least half of the measurement period needs to be in the annual cold season when heating is used. In addition, the measurements need to be taken during a period when the number of consecutive days during which the premises are unoccupied does not exceed 20%.

A–10.

If the sampling duration does not cover an entire year, the best time to perform sampling needs to be determined; this can be different from that derived from the normal seasonal behaviour in buildings (e.g. due to specific variations in natural or mechanical ventilation in underground workplaces, times with higher work capacity and workplaces with different occupancy factors during specific seasons, such as tourist sites and schools).

Location and number of radon measurement points

A–11.

The evaluation of the annual average activity concentration of radon may involve measurements in different zones within the workplace. Passive detectors are relatively inexpensive. Therefore, it can be cost effective to take measurements in several locations within the workplace to establish a more targeted protection strategy. Where a workplace consists of different types of working area, each area could be considered separately for the purpose of radon measurements.

A–12.

Basement or ground floor rooms are likely to have the highest radon concentrations. It is not necessary to carry out measurements in areas that are unoccupied or are occupied infrequently. An infrequently occupied area is generally not expected to be used by individuals for more than 50 hours per year [A–2].

A–13.

The radon activity concentration can vary significantly between adjacent buildings. Consequently, it is usually necessary to take measurements in each separate building. For multi‑storey buildings, measurements taken on the ground floor and in basements (all levels) are the most important for assessing compliance with the reference level for workplaces. Nevertheless, it may also be useful to perform some measurements on upper floors (see also Table A–1).

A–14.

The locations of radon detectors need to be representative in terms of the exposure of workers. Usually, the detectors are not placed in enclosed spaces or directly in a source of fresh air, such as a ventilation intake. Ideally, detectors are placed in the breathing zone of workers but, in any case, at least one metre above the floor, away from heat sources and at least 20 cm from walls or openings.

Radon detectors are not to be moved during the measurement period. Several ways to ensure secure positioning of detectors may be used, for example, tamper-proof tape.

A–15.

As part of the radon measurement protocol, the details of the measurement location and the dates on which measurement started and ended are recorded. Locations in workplaces to be considered for measurement of radon include: (a) Rooms or areas that are occupied on a regular basis for more than four hours a day; (b) All underground rooms and areas that could be used by individuals for more than 50 hours per year (about one hour per week); (c) Any other rooms or areas where there are reasons to believe that radon levels might be of concern.

A–16.

References [A–3, A–4] provide examples of guidance and methodologies used for radon measurements in two Member States. Reference [A–2] contains guidelines for the locations for radon measurements in industrial workplaces. Table A–1 summarizes the guidelines on the number of radon detectors needed. According to Ref. [A–2], the rooms and areas considered for measurement are those that are occupied on a regular basis for more than four hours a day. Other areas to be considered include underground basement rooms or areas where there is a significant risk of radon and that could be used by individuals for more than 50 hours per year (about one hour per week).

Comparing radon measurement results with the reference level

A–17.

The measurement protocol is expected to describe how to evaluate the results of radon measurements in the workplace against the reference level. This includes determining the annual average radon activity concentration to compare directly with the reference level. The protocol may also define whether the evaluation is conducted for the entire workplace, whole buildings or defined areas, whether a reliable estimate of the annual average activity concentration can be obtained with short duration measurements, and how to ensure that any uncertainties are acceptable.

Repeated or follow-up measurements

A–18.

The need to repeat measurements of radon levels in the workplace is normally specified by the regulatory body or other relevant authority. In some cases, it may be necessary to perform follow-up measurements in the workplace, for example using active instruments to evaluate exposure during and outside working hours.

Measurements of radon progeny

A–19.

Measurements of radon progeny are usually more complicated than measurements of radon gas. Radon progeny are mainly attached to fine dust particles in air and have relatively short half-lives. As such, they need to be sampled and analysed relatively quickly (within approximately 30 minutes). This is normally done by taking an air sample through a filter and then analysing the filter.

A–20.

The measurement of radon progeny generally involves active air sampling in which a known volume of air is drawn through a filter. The alpha or beta radiation emitted from radon progeny in the filter is counted during or after sampling. Some methods determine gross activity, while others determine the concentrations of individual radon progeny. For gross alpha counting, the detection is often done simply with a scintillator disc that is mounted on a photomultiplier tube and placed a short distance from the filter at atmospheric pressure. Alpha spectroscopy can be used to determine the activity of individual radon progeny.