Approach to assessing the certainty of evidence in environment and health – the case of WHO global air quality guidelines

Abstract

BACKGROUND AND AIM: The World Health Organization, through the European Centre for Environment and Health, has coordinated a number of evidence synthesis and guidelines, including the recent work on global air quality guidelines (AQGs). The process, described in the WHO Handbook for Guideline Development, involves several groups of experts and requires adherence to methods and procedures. Central is the approach to assessing the certainty of the evidence, retrieved and synthesized through systematic reviews. METHODS: For that purpose, and to develop recommendations, WHO uses the Grading of Recommendations Assessment, Development and Evaluation (GRADE) framework. GRADE has been developed to standardize the approach to judging the certainty of the effects of interventions. While widely used in clinical medicine, its application in the domain of exposures is challenging. In GRADE, the starting point is the study design, with randomized controlled trials considered the ‘golden standard’ and the certainty of observational studies is downgraded. For environmental factors, the evidence base is diverse and with few exceptions does not involve a randomized exposure. Another challenge is to summarize the evidence coming from different lines, involving epidemiological, animal, mechanistic and human clinical studies. Although different groups have adapted the approach for exposures, there is no consensus among experts. RESULTS:Therefore, during the development of the AQGs, a Working Group was established to adapt GRADE to assess the certainty of the evidence from the commissioned systematic reviews. Likewise, another Working Group developed a specific tool to judge risk of bias in individual studies included. The work adaptation benefitted from previous experiences in applying GRADE in occupational and environmental health, as well as specific expertise in air pollution epidemiology. CONCLUSIONS:These tailored approaches, resulted in the methodological adaptations that enhanced comparability of the judgements, when assessors consider the same arguments in a similar manner. However, further work to fine-tune evidence synthesis methods are warranted. KEYWORDS: WHO guidelines, environmental exposure, GRADE

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