Elderly intensive care unit patients are at greater risk of infection if their nurses work more overtime and the units are understaffed, according to a new study by researchers at Columbia University School of Nursing.

Conversely, there is a lower risk of hospital-acquired infections if the units have high nurse staffing levels. Researchers reviewed outcomes for more than 15,000 patients in 51 hospital ICUs. The average ICU staffing level was 17 registered nurse hours per patient per day.

HAIs are the sixth leading cause of death in the United States. The study appears in the June issue of the journal Medical Care.