Impact of urban air ultrafine particles on embryonic development and their association to transport emissions
Airborne ultrafine particles (UFP) constitute a potential risk factor for human health, being transport one of the most relevant sources, particularly in urban environments. We collected UFP from outdoor sites in Barcelona and Valencia (Spain) selected to distinguish between the contributions of road traffic, trains, ships, and planes.  Zebrafish embryos were exposed to organic extracts from the collection filters, and their transcriptomes were analysed by RNA deep sequencing. Functional analysis of 2,941 significantly affected transcripts revealed a strong enrichment in functions related to general embryotoxicity, like induction of degradation pathways (endocytosis, necroptosis), and inhibition of development- and cell division-related genes. This embryotoxicity-related transcriptional response was linked to elevated  concentrations of UFP, sulfur, and other inorganic species in the air samples, and it was particularly strong in UFP-rich airport samples. In addition,  the functional analysis also identified a relatively small subset of genes configuring a typical  dioxin-like response, traditionally associated with exposure to airborne combustion particles, and that was linked to PAH-rich emissions from harbour and traffic samples. Consistently, relative abundance of the implicated genes showed  a strong correlation with high concentrations of polycyclic aromatic hydrocarbons (PAHs) in the extracts. Our findings indicate that the parameters currently used to assess air quality overlook the role of UFP in total airborne toxicity and, consequently, they do not adequately protect human populations.