Emergency departments probably have seen a lot of fireworks injuries within the last few weeks, and, whether clinicians realized it or not, the most serious ones probably resulted from certain types of explosives.

A new study, published in The American Journal of Emergency Medicine, finds that not all pyrotechnics are created equal when it comes to sending users for emergency care, however.

Even though shell-and-mortar-style fireworks are legal in many areas, they caused nearly 40% of fireworks-related injuries resulting in hospitalization, report researchers from the University of Washington’s Harborview Medical Center.

The study notes that shells, which are sphere-shaped aerial explosives designed to be manually thrown or launched from a mortar, are legal under both federal and Washington state law.

About 10,500 patients are treated annually in hospital EDs for fireworks-related injuries, according to the U.S. Consumer Product Safety Commission, and that number has remained relatively stable since 1999.

This study sought to determine the frequency and cause of severe fireworks injuries, defined as those requiring hospital admission.

For the report, the research team reviewed records on 294 patients admitted to Harborview Medical Center between 2005-2015 for severe fireworks injuries. Males accounted for 90% of the patients, and the average age was 24. The greatest proportion of injuries (39%) was related to shells or mortars, especially among adults, where 86% of those presenting to the ED were hurt by those devices.

Proportionally, there were more injuries from rockets in children (44%), and more injuries from homemade fireworks in teenagers (34%).

During the 2005-2015 study period, 119 (40%) patients were admitted who did not undergo surgery, 163 (55%) patients required both admission and surgery, 12 (5%) patients underwent outpatient surgery, and two patients (< 1%) died.

“Brain, face, and hand injuries were disproportionately represented in the shells/mortars group,” study authors report. “Seventy percent of globe-injured patients experienced partial or complete permanent vision loss. Thirty-seven percent of hand-injured patients required at least one partial or whole finger/hand amputation. The greatest proportion of eye and hand injuries resulting in permanent impairment was in the shells/mortars group, followed by homemade fireworks.”

Researchers emphasize that their findings were important because they were able to determine specific injury patterns, with shells/mortars presenting the most danger and potentially leading to greater permanent impairment from eye and hand injury.