In 2004, California enacted a nurse-to-patient ratio law. To this day, California is the only state with a nurse-to-patient ratio law. On most hospital wards, the law mandates a minimum ratio of one nurse for every five patients; within Intensive Care Units, the ratio is one to two (1:2). These mandated ratios are typically higher than the prevailing ratios prior to 2004. In fact, nurse employment rose approximately 15 percent after 2004 as a result of the law. The intent of the law was to improve care for patients and although no consensus has yet been reached, studies have shown that the law has improved patient care in a variety of domains.

My colleagues and I addressed a different research question: Could the law have improved safety for the nurses themselves? We found that it did; occupational injury and illness rates dropped over 30 percent. This is important, in part, because the nursing occupation generates more occupational injuries to women than any other occupation. We used data on work injuries resulting in at least one day of work loss among registered nurses and licensed practical nurses. These data were drawn from the Bureau of Labor Statistics Survey of Occupational Injuries and Illnesses. This Survey, authorized by the OSHA law, has been collecting information on occupational injuries and illnesses from 160,000 to over 250,000 private establishments since the1970s.

Our data were annual injury and illness rates (cases per total number of employed) from 1999 through 2009. We used the difference-in-differences approach. The rates before 2004 were subtracted from the rates after 2004 within California. This California subtracted difference was then compared to a similar subtracted difference before and after 2004 for the other 49 states. In scientific language, California was the “intervention group” and the other 49 states were the “control group.” The subtracted difference for California was more than 30 percent larger than the difference for the other 49 states for both registered nurses and licensed practical nurses.

There are many mechanisms whereby greater numbers of nurses taking care of the same number of patients may result in fewer nurse injuries. For example, many nurses injure their backs they try to move patients. The nurse may weigh 130 pounds and the patient weighs 200 pounds. Back injuries have been shown to be much less frequent when two nurses lift a patient, rather than one.

Although the study did not address costs, it is likely that the law resulted in lower worker’s compensation costs because employment grew by 15 percent whereas injuries per employed nurse dropped by 30 percent.

Our results suggest that other state legislatures  should consider the benefits to nurse safety that could result from enacting laws setting minimum nurse-to-patient ratios.