Fig. 1

Literature search methodology. Papers were selected for inclusion in the review by combining each of the search boxes in the figure with the Boolean operator “AND”, across four databases. The 203 papers yielded by this initial search were refined to include only those of relevance to underground railways and health, and supplemented by relevant papers used in the review of Xu and Hao [13] and also the IMPROVE study. This yielded the 27 papers which form the core body of literature for this review. Further studies have been included and cited where appropriate