Comparison of endotoxin levels and Gram-negative bacteria under different conditions in microbial laboratories and a biowaste site

► Endotoxin levels were significantly higher in labs that did not have an HVAC system, that had GNB present, and that were sampled on rainy days. ► Endotoxin levels are higher, although not significant, when an open biowaste box is present. ► Special attention is given to open biowaste boxes and proper control relative humidity to improve the indoor air quality of microbial laboratories.