Cross-Species Evaluation of Bioaccumulation Thresholds for Air-Breathing Animals

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.