title: Expert elicitation on ultrafine particles: likelihood of health effects and causal pathways.
abstract: BACKGROUND
  Exposure to fine ambient particulate matter (PM) has consistently been associated with increased morbidity and mortality. The relationship between exposure to ultrafine particles (UFP) and health effects is less firmly established. If UFP cause health effects independently from coarser fractions, this could affect health impact assessment of air pollution, which would possibly lead to alternative policy options to be considered to reduce the disease burden of PM. Therefore, we organized an expert elicitation workshop to assess the evidence for a causal relationship between exposure to UFP and health endpoints.
  
  
  METHODS
  An expert elicitation on the health effects of ambient ultrafine particle exposure was carried out, focusing on: 1) the likelihood of causal relationships with key health endpoints, and 2) the likelihood of potential causal pathways for cardiac events. Based on a systematic peer-nomination procedure, fourteen European experts (epidemiologists, toxicologists and clinicians) were selected, of whom twelve attended. They were provided with a briefing book containing key literature. After a group discussion, individual expert judgments in the form of ratings of the likelihood of causal relationships and pathways were obtained using a confidence scheme adapted from the one used by the Intergovernmental Panel on Climate Change.
  
  
  RESULTS
  The likelihood of an independent causal relationship between increased short-term UFP exposure and increased all-cause mortality, hospital admissions for cardiovascular and respiratory diseases, aggravation of asthma symptoms and lung function decrements was rated medium to high by most experts. The likelihood for long-term UFP exposure to be causally related to all cause mortality, cardiovascular and respiratory morbidity and lung cancer was rated slightly lower, mostly medium. The experts rated the likelihood of each of the six identified possible causal pathways separately. Out of these six, the highest likelihood was rated for the pathway involving respiratory inflammation and subsequent thrombotic effects.
  
  
  CONCLUSIONS
  The overall medium to high likelihood rating of causality of health effects of UFP exposure and the high likelihood rating of at least one of the proposed causal mechanisms explaining associations between UFP and cardiac events, stresses the importance of considering UFP in future health impact assessments of (transport-related) air pollution, and the need for further research on UFP exposure and health effects.
authors.affiliation: Dutch National Institute for Public Health and the Environment (RIVM), Bilthoven, the Netherlands. anne.knol@rivm.nl.
authors.forename: Anne B
authors.initials: AB
authors.lastname: Knol
authors.forename: Jeroen J
authors.initials: JJ
authors.lastname: de Hartog
authors.forename: Hanna
authors.initials: H
authors.lastname: Boogaard
authors.forename: Pauline
authors.initials: P
authors.lastname: Slottje
authors.forename: Jeroen P
authors.initials: JP
authors.lastname: van der Sluijs
authors.forename: Erik
authors.initials: E
authors.lastname: Lebret
authors.forename: Flemming R
authors.initials: FR
authors.lastname: Cassee
authors.forename: J Arjan
authors.initials: JA
authors.lastname: Wardekker
authors.forename: Jon G
authors.initials: JG
authors.lastname: Ayres
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Borm
authors.forename: Bert
authors.initials: B
authors.lastname: Brunekreef
authors.forename: Kenneth
authors.initials: K
authors.lastname: Donaldson
authors.forename: Francesco
authors.initials: F
authors.lastname: Forastiere
authors.forename: Stephen T
authors.initials: ST
authors.lastname: Holgate
authors.forename: Wolfgang G
authors.initials: WG
authors.lastname: Kreyling
authors.forename: Benoit
authors.initials: B
authors.lastname: Nemery
authors.forename: Juha
authors.initials: J
authors.lastname: Pekkanen
authors.forename: Vicky
authors.initials: V
authors.lastname: Stone
authors.forename: H-Erich
authors.initials: HE
authors.lastname: Wichmann
authors.forename: Gerard
authors.initials: G
authors.lastname: Hoek