Data extraction

Extraction of data was performed systematically by summarizing information on author, publication year, country, study design, endotoxin measurement techniques, spirometry measurements, questionnaires and confounders in overview tables.

Studies were categorized according to characteristics of the sample population (i.e., occupationally exposed subjects, respiratory disease patient groups or general population). Results are presented by individual study, since the studies were too heterogeneous in terms of endpoint measurement and presentation of endotoxin exposure levels, population samples, settings, reported outcomes and data analysis techniques to compare the results. Therefore, a narrative synthesis was performed and a best evidence synthesis was conducted for suitable outcome variables (see data synthesis). Also, forest plots were constructed (using R, version 3.3.2) to improve readability and comparability of the results.