What does "ND" mean in environmental testing? Please let the great god know, thank you.

ND stands for lower limit. Use specific instruments and methods for monitoring. For example, its lower limit is 0. 1mg/m3. In fact, as long as it is less than or equal to 0. 1mg/m3, the monitoring data are all 0. 1mg/m3. In order to show that the lower limit has been reached in environmental protection, it is expressed by ND. These are what I saw in official website. Go directly to official website for details ~