Do you know what the ambient gamma radiation levels in Hong Kong are? If you check the real time radiation level in Hong Kong on the Hong Kong Observatory’s website, the ambient gamma radiation dose rates will usually be around 0.1 microsievert per hour (Figure 1). The difference of radiation levels recorded by various monitoring stations is mainly due to environmental difference and soil nature in those places. A remark on the webpage indicates that the hourly mean ambient gamma radiation dose rate in Hong Kong may vary between 0.06 and 0.3 microsievert per hour. Why can the radiation levels be doubled or more than those usually observed, says 0.3 microsievert per hour? 

Figure 2 shows the ambient gamma radiation dose rates recorded by various monitoring stations on 19 April 2023. It can be seen that there was once a significant increase in radiation levels after 10:30 a.m. What happened?

Let’s look at the radar images in that period (Figure 3). We can see that a band of thundery showers was moving eastwards and affecting Hong Kong. Do you notice that after the rain band passed the monitoring stations, the radiation levels recorded by the stations increased subsequently?

The ambient gamma dose rate can be affected by rainfall. This is because rain can wash down radon progenies in air and results in a temporary rise of gamma dose rates on ground. In this case, will continuous rainfall keep radiation levels rising? The answer is no. The elevated radiation levels can last for a few hours, and then may drop to their usual levels or even about 5% below. Rainfall can cause an increase in the ambient gamma radiation levels, mainly due to the presence of lead-214 and bismuth-214, which are the gamma radioactive progenies of radon-222 (i.e. radon) in air^[1]. The rainfall process can wash them down to the ground, increasing the near-ground radiation levels. On the other hand, the half-lives of lead-214 and bismuth-214 are relatively short, about 27 minutes and 20 minutes respectively. Their numbers will decrease over a short period of time due to decay, and the radiation levels will gradually drop to the usual levels. However, accumulated rainfall may cause stagnant water in the environment which has shielding effect on radiation. Rain can also increase soil moisture content, which hinders the emission of radon gas from the soil to the atmosphere^[2]. When most of the radon progenies scavenged to the ground have decayed, depending on the amount of stagnant water and soil moisture in the environment at that time, the radiation levels may drop below their usual levels.

So please don’t panic when you see a slight increase in the ambient gamma radiation levels. The rise is usually due to weather and environmental changes^[3].