In air-breathing organisms, an organic chemical's susceptibility to elimination via urinary excretion and respiratory exhalation can be judged on the basis of the octanol-water partition ratio (KOW) and the octanol-air partition ratio (KOA), respectively. Current regulations specify that chemicals with KOW values of <102 and KOA values of <105 may be screened as non-bioaccumulative in air breathers. Here we used a model-based approach to evaluate whether these thresholds are consistent with a biomagnification factor of 1 for 141 different mammals, birds, and reptiles. Animals with lower rates of respiration (e.g., manatees and sloths) and those ingesting high-lipid diets (e.g., polar bears and carnivorous birds) were predicted to be able to biomagnify persistent chemicals with KOA values of <105. This was also observed for several temperate reptiles due to their lower respiration rates and internal temperatures. Protective KOA thresholds were determined to be <104.85 for mammals, <104.60 for birds, <104.60 for reptiles at >25 °C, and <103.95 for reptiles at ≤25 °C. For all animals, urination alone was not efficient to prevent the biomagnification of any organic chemical. For chemicals with KOW values of <101, we found that biomagnification of persistent chemicals was constrained by the water-air partition ratio (KWA) rather than KOA. Differences in physiology may need to be considered in bioaccumulation assessments of air-breathing species.
Keywords: air breathers; bioaccumulation; biodiversity; biomagnification; cross-species evaluation; respiration.