临界物质

例句：

1. 1. The concentration of the critical substance is too low to be detected.
   临界物质的浓度太低，无法检测到。
2. 2. The researchers found that the threshold for detecting the critical substance was very high.
   研究人员发现，检测临界物质的阈值非常高。
3. 3. The study focused on the critical substances that could lead to environmental pollution.
   该研究关注可能导致环境污染的临界物质。