Different decreasing rates of chemical threshold concentrations can be explained by their toxicokinetic and toxicodynamic characteristics

To protect ecosystems, threshold concentrations (e.g., HC₅) for chemicals are often derived using the toxicity data obtained at fixed times. Since the toxicity (e.g., LC₅₀) usually decreases with exposure time, the threshold concentrations are expected to be time-dependent, giving rise to the uncertainty in the chemical environmental criteria. Here, using the LC₅₀ data with at least 3 different exposure durations (24, 48 and 96 h) for compounds, we explored the time evolutions of HC₅ across 20 chemicals. Results showed that all chemical's HC₅ decreased with time, but their decreasing rates of HC₅ (k) are significantly different: for some chemicals the k are lower than 0.001 (e.g., methoxychlor and dieldrin), while for some chemicals the k are higher than 0.05 (e.g., PCP and aldicarb). Furthermore, we found that k is negatively related to the bioconcentration factors (BCF), and positively related to the damage recovery rates (k_R). Our work demonstrated that time is an important source of the ecological threshold uncertainty, and this uncertainty is associated with chemical-specific toxicokinetic and toxicodynamic characteristics. We recommend that to effectively protect the ecological communities, higher assessment factor should be adopted in deriving the acute environmental criteria for these chemicals with high BCF and low k_R, fluoranthene and diazinon.