This paper aims to serve as an introduction to the Special Issue in Environment International entitled “Update of the WHO Global Air Quality Guidelines: Systematic Reviews”. The article has two main objectives. One is to provide the context to this Special Issue, related to (a) policy context, overall exposure to air pollution, and burden of disease attributable to air pollution, and the other is to describe (b) the WHO guideline development process, with special emphasis on the systematic reviews. In particular, this paper presents the systematic reviews and other supporting evidence that was used and discussed during the process and summarizes important methodological information about the approaches taken to conduct the systematic reviews. These approaches include the definition of population, exposure, comparator, outcomes and study design (PECOS) questions, the assessment of the risk of bias in individual studies and the assessment of the overall certainty of the evidence. In summary, the new WHO global air quality guidelines are informed by the best available scientific evidence covering a vast number of research papers published until September 2018, and appraised by experts and stakeholders in the field of air quality. However, research gaps remain and, therefore, further research is warranted.

本文旨在作为对环境国际特刊的介绍题为“世卫组织全球空气质量指南的更新：系统审查”。这篇文章有两个主要目标。一个是提供本期特刊的背景，与 (a) 政策背景、总体空气污染暴露和空气污染引起的疾病负担有关，另一个是描述 (b) WHO 指南制定过​​程，包括特别强调系统评价。特别是，本文介绍了系统评价和在此过程中使用和讨论的其他支持证据，并总结了有关进行系统评价所采用方法的重要方法学信息。这些方法包括人口、暴露、比较、结果和研究设计 (PECOS) 问题的定义，个别研究的偏倚风险评估和证据的总体确定性评估。总之，新的世卫组织全球空气质量指南以现有最佳科学证据为依据，涵盖截至 2018 年 9 月发表的大量研究论文，并由空气质量领域的专家和利益相关者进行评估。然而，研究差距仍然存在，因此，进一步的研究是必要的。