Inhale, exhale: Why particulate matter exposure in animal models are so acute?

Ecotoxicological studies that try to describe the effects of particulate matter (PM) on human health are important in order to gain a deeper understanding of their effects in disease outcomes. Because exposure protocols are not easily comparable, evaluating human PM exposure is a difficult task. Thus, interpreting ambiguous or conflicting results from different experiments could lead to misleading conclusions about the true nature of PM effects. To address these issues, we compiled a collection of relevant research articles in order to compare present PM exposure methods and extract data related to concentration, inhalation rates (IR), and doses. We also compare the experimental exposure levels reported in these articles to PM levels around the world. In particular, our dataset covers reported results from 75 research articles. To allow for comparison between protocols, we used this data to fit a normalization equation that depends upon concentration, exposure time, dose, inhalability, and physiological parameters. Based on the collected research papers, instillation is the prevalent exposure method. Also, the median PM IR from these experiments is three orders of magnitude higher than the PM IR found in environmental conditions (EAP). Experiments employing inhalation of concentrated PM show IR results that are two orders of magnitude higher than EAP; these results are cause for concerns, since the PM exposure were acute, sudden, and higher than the worst-case exposure scenarios reported by the world megacities. We also found that different PM exposure protocols are sources for the observed variability in physiological response results found from animal models. We discuss these findings and make suggestions for future exposure methodologies. Such considerations should be valuable for quantifying PM exposure in disease outcomes.