Background: Exposurei 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 assessmentii 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 peernomination
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 thelikelihood 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 mediumi 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.
Conclusion: 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.