Expert elicitation of health risks of Ultra Fine Particles

Introduction

This case study is an example of Expert Elicitation for the case of Health Effects Related to Exposure to Ultrafine Particles. The elicition addressed likelihood of causality and causal pathways and concentration response functions for Ultrafine Particles and all-cause mortalilty and hospital admissions.
Exposure to fine ambient particulate matter (PM) has consistently been associated with morbidity and mortality. However, the association of health effects from exposure to ambient ultrafine particles (UFP) is still under debate. Therefore, an expert elicitation workshop was organized to assess the evidence for a causal relationship between exposure to UFP and health endpoints. 
Toxicological studies have provided evidence of the toxicity of ultrafine particles (UFP), but epidemiological evidence for health effects of UFP is limited. No quantitative summary currently exists of concentration response functions for UFP, that can be used in health impact assessment. A second goal of the workshop thus was to specify concentration response functions for ultrafine particles in urban air including its uncertainty through an expert panel elicitation.

Method

The workshop focused on:
1) the likelihood of causal relationships with key health endpoints, and
2) the likelihood of causal pathways for cardiac events.

Selected through a systematic peernomination procedure, twelve European experts (epidemiologists, toxicologists and clinicians) attended the workshop. 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 Intergovernmental Panel on Climate Change.
Using individual ratings supplemented with group discussion, probability distributions of effect estimates were obtained for all-cause  mortality and cardiovascular and respiratory hospital admissions.

Results

The likelihood of an independent causal relationship between increased short-term UFP exposure and increased all-cause mortality, cardiovascular and respiratory hospital admissions, 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 highest likelihood for the pathway involving respiratory inflammation and subsequent thrombotic effects; translocation of particles to the blood and subsequent effects on the autonomic nervous system and cardiac rhythm was considered the least likely pathway towards cardiac events.

Experts were willing to quantify effects of UFP on all-cause mortality and to a lesser extent hospital admissions. Between experts substantial differences in the estimated UFP health effect and its uncertainty were found. The mean of the central estimate of the percentage decrease in all-cause mortality with a 1,000 p/cm3 permanent decrease in UFP concentration was 0.43%. The lack of studies on longterm exposure was rated as the most important factor contributing to the overall uncertainty. Most experts felt that toxicity of UFP differed depending on source and/or composition. Effects on cardiovascular and respiratory hospital admissions were considered more uncertain.

Conclusion

Overall the results of the expert elicitation indicated that there is medium to high likelihood of health effects associated to UFP exposure, most likely through respiratory inflammation and subsequent thrombotic effects. The results stresses the importance of considering UFP in future Health Impact Assessments of (transport-related) air pollution, and the need for further research on health effects of UFP exposure.

This expert elicitation provides the first estimate of quantitative summary concentration response functions between urban air ultrafine particles and all cause mortality and hospital admissions, along with explicit estimates of their uncertainty.

Documentation of the case

Attached are the workshop program, the briefing book for the participants and the elicitation protocol used during the workshop. The password protected workshop website can be accessed (username ufp password workshop) via http://www.nusap.net/ufp. Two powerpoint presentations are also attached (day 1, day 2)

The results are presented in two papers and a PhD Thesis:

Anne Knol, Jeroen de Hartog, Hanna Boogaard, Pauline Slottje, Jeroen van der Sluijs, Erik Lebret, Flemming Cassee, Arjan Wardekker, Jon Ayres, Paul Borm, Bert Brunekreef, Ken Donaldson, Francesco Forastiere, Stephen Holgate, Wolfgang Kreyling, Benoit Nemery, Juha Pekkanen, Vicky Stone, Erich Wichmann, Gerard Hoek (2009).Expert elicitation on health effects related to exposure to ultrafine particles: likelihood of causality and causal pathways, Part Fibre.Toxicology 6:19
doi: 10.1186/1743-8977-6-19

Gerard Hoek, Hanna Boogaard, Anne Knol, Jeroen de Hartog, Pauline Slottje, Jon G Ayres, Paul Borm, Bert Brunekreef, Ken Donaldson, Francesco Forastiere, Stephen Holgate, Wolfgang G. Kreyling, Benoit Nemery, Juha Pekkanen, Vicky Stone, H.-Erich Wichmann, Jeroen van der Sluijs (2010), Concentration Response Functions for Ultrafine Particles and All-Cause Mortality and Hospital Admissions: Results of a European Expert Panel Elicitation, Environmental Science and Technology, 44 (1), 476–482, DOI: 10.1021/es9021393

The method is elaborated in these two papers:

P. Slottje, J.P. van der Sluijs and A.B. Knol, 2008, Expert Elicitation - Methodological suggestions for its use in environmental health impact assessments. RIVM Letter Report 630004001/2008, National Institute for Public Health and the Environment, Bilthoven, 56 pp.

A.B. Knol, P. Slottje, J.P. van der Sluijs, E. Lebret (2010), The use of expert elicitation in environmental health impact assessment: a seven step procedure, Environmental Health 9:19.