**2. Sources of radon**

Radium 226 decays into radon 222, which in turn decays into its short-lived radioactive daughters in the mine atmosphere. Common sources of radon emissions in underground mines are summarized in **Table 1**.

### **3. Radon monitoring**

The concentration of radon gas is measured in units of picocuries per liter (pCi/L) or becquerels per cubic meter (Bq/m3 ) of ambient air. Due to difficulties in measuring radon gas concentration, potential alpha particles per liter of air are usually measured. The ratio of all the short-lived radon daughters' activity to the


### **Table 1.**

*Common sources of radon in underground uranium mines [3].*

parent radon gas activity is called the equilibrium factor. The equilibrium factor is 1 when both are equal. Radon daughter activities are usually less than the radon activity, and hence, the equilibrium factor is generally less than 1. In artificially ventilated scenarios such as underground mines, the equilibrium factor is in the range of 0.4 to 0.5.

In the United States, radioactivity for radon decay products is measured in terms of Working Level (WL). A WL is defined as the concentration of short-lived radon daughters, representing 1.3 × 105 MeV of potential alpha particle energy while decaying to the stable Pb-210. The worker's prolonged exposure to radon daughters is expressed in Working Level Months (WLM). One WLM is equivalent to 1 WL exposure for 170 hours.

In underground mines as per the Mine Safety and Health Administration (MSHA) regulations, personnel shall not be exposed to air containing concentrations of radon daughters exceeding 1.0 WL. No person shall be permitted to receive


### **Table 2.**

*Radon daughter exposure monitoring [7].*

exposure over 4 WLM (Working Level Months) in any calendar year. In all mines, at least one sample must be taken in exhaust mine air by a competent person to determine whether concentrations of radon daughters are present [7]. **Table 2** provides the radon sampling frequency for uranium and non-uranium mines and households. Gamma radiation surveys shall be conducted annually in all underground mines where radioactive ores are mined. Gamma radiation dosimeters shall be provided to all personnel working in the area where gamma radiation exceeds 2.0 milliroentgens; annual individual gamma radiation exposure shall not exceed 5 Roentgen Equivalent Man [7].
