Photochemical processes are typically not incorporated in screening-level substance risk assessments due to the complexity of modeling sunlight co-exposures and resulting interactions on environmental fate and effects. However, for many substances, sunlight exerts a profound influence on environmental degradation rates and ecotoxicities. Recent modeling advances provide an improved technical basis for estimating the effect of sunlight in modulating both substance exposure and toxicity in the aquatic environment. Screening model simulations were performed for 25 petrochemical structures with varied uses and environmental fate properties. Model predictions were evaluated by comparing the ratios of predicted exposure concentrations with and without light to the corresponding ratios of toxicity thresholds under the same conditions. The relative ratios of exposure and hazard in light vs. dark were then used to evaluate how inclusion of light modulates substance risk analysis. Results indicated that inclusion of light reduced PECs by factors ranging from 1.1- to 63-fold as a result of photodegradation, while reducing PNECs by factors ranging from 1- to 49-fold due to photoenhanced toxicity caused by photosensitization. Consequently, the presence of light altered risk quotients by factors that ranged from 0.1- to 17-fold, since the predicted increase in substance hazard was mitigated by the reduction in exposure. For many structures, indirect photodegradation decreases environmental exposures independently of the direct photolysis pathway which is associated with enhanced phototoxicity. For most of the scenarios and chemicals in the present work, photosensitization appears to be mitigated by direct and indirect degradation from sunlight exposure.

中文翻译：

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水生系统中的直接和间接光降解可减轻选定石化结构筛选级物质风险评估中的光敏毒性


由于阳光共同暴露建模的复杂性以及由此产生的对环境命运和影响的相互作用，光化学过程通常不纳入筛选级物质风险评估。然而，对于许多物质来说，阳光对环境降解率和生态毒性有着深远的影响。最近的建模进展为估计阳光在调节水生环境中的物质暴露和毒性方面的影响提供了改进的技术基础。对具有不同用途和环境归宿特性的 25 个石化结构进行了筛选模型模拟。通过比较相同条件下有光和无光下的预测暴露浓度比率与相应的毒性阈值比率来评估模型预测。然后使用光与暗中的暴露和危害的相对比率来评估光的包含如何调节物质风险分析。结果表明，由于光降解，加入光使 PEC 减少 1.1 至 63 倍，而由于光敏化引起的光增强毒性，使 PNEC 减少 1 至 49 倍。因此，光的存在使风险商数改变了 0.1 至 17 倍，因为暴露量的减少减轻了预期的物质危害增加。对于许多结构而言，间接光降解可减少环境暴露，而与直接光解途径无关，而直接光解途径与增强的光毒性有关。对于目前工作中的大多数场景和化学品，光敏作用似乎可以通过阳光照射的直接和间接降解来减轻。