Fatalities in football are rare. A 2013 study of high school and college football players split fatalities into two types: direct fatalities, defined as those caused by "trauma from participation in a sport resulting in a brain injury, cervical fracture, or intra-abdominal injury" and indirect fatalities, defined as those resulting from external factors such as "cardiac failure, heat illness, sickle cell trait , asthma, or pulmonary embolism". The study found that, on average, there are 4 direct fatalities and 8.2 indirect fatalities among high school and college players per year, making indirect fatalities more than twice as common as direct fatalities.