Background: The effectiveness of an airflow system in preventing entrainment of particles carrying micro-organisms from the periphery of an operating room (OR) or instrument layup room (ILR) is affected by many variables. It is suspected that differences in the design of the systems affect the effective protection ratio (EPR): the ratio of the size of the protected area to the surface area of the supply canopy. However, no analysis has yet been done to determine which design variables have a significant effect on this ratio. Aim: To evaluate which design variables have a significant effect on the performance of airflow systems (EPR) in ORs and ILRs. Methods: All general and teaching hospitals in the Netherlands (N ¼ 77) were asked to provide data from their standardized (at-rest measurement method) compulsory systems assessment reports for ORs and ILRs. Nineteen hospitals (25%) with a total of 22 hospital sites supplied information of sufficient completeness and homogeneity, resulting in measurement data for 101 ORs and 23 ILRs. This dataset was analysed using Statistical Package for Social Sciences. Findings: For ORs, important predictors for the EPR were: shape of the canopy; air speed under the supply canopy; height of the canopy screen; type of system; and size of the canopy. These significant predictors (P < 0.05) explain 48% of the outcome in the dataset. For ILRs, significant predictors for the EPR were: the position of exhaust air terminals; height of the canopy screen; and size of the canopy. These significant predictors explain 66% of the outcome in the dataset. Conclusion: On the basis of the dataset available for analysis, it is concluded that the ratio of the size of the protected area to the surface area of the supply canopy (EPR) improves with the presence and the height of a screen around the canopy, the surface area of the supply canopy, and the air speed of the supply air under the canopy. This information can be used as guidance for the future design of unidirectional displacement airflow systems.