Chicago ? Amid concerns about a nationwide nurse shortage, a new study found that how many patients a nurse has to care for can be a matter of life or death.

Researchers found that patients had a greater chance of dying following surgery in hospitals where the nurses had to take care of more patients, according to the study in today’s Journal of the American Medical Assn.

A heavy workload also meant nurses were more likely to be burned out and unhappy with their jobs.

In the study, University of Pennsylvania researchers found that each additional patient in a nurse’s workload translated to about a 7 percent increase in the likelihood the patient would die within 30 days of admission. For example, the difference between four and six patients per nurse translated to a 14 percent increase in mortality, while the difference between four and eight patients increased the likelihood of dying by 31 percent.

To the American Nurses Assn., the study confirms what nurses have been saying all along: They do make a difference.

“When a nurse can’t be there to see the subtle changes that happen in patients and intervene when those changes are subtle, then patients are going to get into real serious crises and by the time they are in those crises we may not be able to save them,” said Barbara Blakeney, president of the ANA.

The study does not suggest how many nurses are needed to care for patients or whether there is a maximum number of patients per nurse hospitals should never exceed.