The 1-hour PM2.5 concentrations reflect the PM2.5 levels averaged over one hour, and can give you an indication of the current air quality. However, the 1-hour PM2.5 concentration levels can be volatile and tend to fluctuate over the day especially during periods of transboundary haze. Short-term fluctuations will also be very heavily influenced by weather conditions. For example, there could be a short period of transiently high PM2.5, only to improve rapidly in the next one to two hours. Therefore, if you wish to use the 1-hour PM2.5 concentration, it is best used as a guide to adjust your immediate activities. During haze episodes, health advice will be based on the 24-hour PSI. It is a better reflection of the total exposure of an individual to PM2.5 levels, and scientific and epidemiological studies on the health effects of particulate matter have been based on the 24-hour duration of exposure.