Injury is not an “accident” but rather a disease, much like malaria, tuberculosis and other public health problems, or cancer and heart disease. Injury, like other diseases, has variants such as blunt or penetrating. It has degrees of severity, rates of incidence, prevalence, and mortality that can differ by race and other sociodemographic factors. Injuries have a predictable pattern of occurrence related to age, sex, alcohol and other drugs, and again, sociodemographic factors, among others. They also have a predictable prognosis, based on age, sociodemographic factors, as well as injury severity.

This characterization of injury as a disease is an important one, and a matter of more than just semantics. It is only when public health concepts are applied to this disease of injury that it, like other public health diseases, can be controlled to a socially acceptable level. The first step after its recognition as a disease is to characterize the disease such that control strategies can be applied. Epidemiology is the study of patterns of disease occurrence in human populations and the factors that influence these patterns.¹ Thus, the majority of injury epidemiology relates to describing specific populations and the factors that influence injury occurrence in these populations.

Descriptive epidemiology refers to the distribution of disease over time, place, and within or across specific subgroups of the population. It is important for understanding the impact of injury in a population and identifying opportunities for intervention. The burden of injury can be described as the most common, most fatal, most debilitating, or most costly within a specific population.

Analytic epidemiology, in contrast, refers to the more detailed study of the determinants of observed distributions of disease in terms of causal factors. The epidemiological framework traditionally identifies these factors as related to the host (ie, characteristics intrinsic to the person), the agent (physical, chemical, nutritive, or infectious), and the environment (ie, characteristics extrinsic to the individual that influence exposure or susceptibility to the agent). The environment can be physical or sociocultural. The importance of this epidemiological approach is the direction it gives to injury prevention efforts as well as directing areas requiring further research.

Injuries can result from acute exposure to physical agents such as mechanical energy, heat, electricity, chemicals, and ionizing radiation in amounts or rates above or below the threshold of human tolerance.² The transfer of mechanical energy accounts for more than three quarters of all injuries.³ The extent and severity of injury is largely determined by the amount of energy outside the threshold of human tolerance. Both the exposure to energy and the consequences of that exposure are greatly influenced by a variety of factors both within and beyond individual or societal control.⁴

The concepts of the public health approach applied to injury control seek to modulate factors related to the host and agent and/or their interactions within the environment utilizing a number of strategies. These strategies encompass engineering, education, the enactment and enforcement of laws, and economic incentives and disincentives.

The public health approach as it applies to injury was first conceptualized by William Haddon in the late 1960s.² He developed and promulgated a phase-factor matrix that incorporated the classic epidemiological framework of host, agent, and environment in a time sequence that encompasses three phases: pre-event, event, and post-event. Factors related to the host, agent, or environment in the pre-event phase determine whether the event will occur (eg, motor vehicle crash). Factors in the event phase determine whether an injury will occur as a result of the event and the degree of injury severity. Factors in the post-event phase influence the outcome from, or consequences of, any injuries of any severity that do occur. An example of the Haddon Matrix applied to an actual injury event is depicted in Table 2-1.

Although the Haddon Matrix is the foundation of injury epidemiology, it is not enough to direct robust injury prevention and control efforts. The addition of potential control strategies to the matrix in a three-dimensional fashion results in an “injury control cube,” suggesting that injury prevention and control are not unidimensional or unifactorial and that the greater the number of sections of the “cube” that are addressed, the greater the control of the injury event. For example, gun control laws focus on only the agent, in the pre-event phase, using a legislative strategy (Fig. 2-1). However, there are many other counter measures that can be applied in other phases and to the host or environment.

Injuries rank fourth as a cause of death for all age groups in this country, and have consistently held that place for many years. It is the leading cause of death among children, adolescents, and young adults ages 1–44 (Table 2-2).³ In 2013, 192,945 people persons died in the United States as a result of an injury, up from approximately 150,000 in 2009 and resulting in an age-adjusted injury rate per 100,000 population of 58.53. The predominance of injury deaths among the young results in another measure of the burden of injury, years of productive life lost. This measure makes the assumption that individuals are most productive to society before the age of 65; given the ever-increasing length of productive life, this is an often incorrect assumption. Nevertheless, it does give some measure of comparison of the effect of various causes of mortality. In 2013, all causes of death contributed to over 11 million years of productive life lost. Deaths from injury were responsible for 31.2%, more than any other individual category. Unintentional injury was responsible for 19%, suicide for 7%, and homicide for 5% of total years of productive life lost.

From 2002 to 2010, the total trauma-related mortality decreased by 6%. However, mortality trends differed by mechanism. There was a 27% decrease in the motor vehicle-associated death rate associated with a 20% decrease in motor vehicle collisions, 19% decrease in the number of occupant injuries per collision, lower injury severity, and improved outcomes at trauma centers (Fig. 2-2). While firearm-related mortality remained relatively unchanged, mortality caused by firearm suicides increased, whereas homicide-associated mortality decreased. In contrast, fall-related mortality increased by 46%.⁵

The timing of trauma deaths, classically described as trimodal, has changed due to advances in resuscitation and ICU that have essentially eliminated the last peak of deaths from multisystem organ failure.^6,7,8 The majority of all deaths still occur within minutes of the injury, either at the scene prior to arrival of emergency medical service (EMS), en route to the hospital, or in the first hours of care. These immediate deaths are typically the result of massive hemorrhage or severe neurological injury. The second peak of the bimodal death distribution occurs within several hours to days of the event, and is due primarily to central nervous system (CNS) injury (Fig. 2-3).^7,8

Deaths represent only one small aspect of the injury disease burden. Each year, almost 31 million people suffer a nonfatal injury; the vast majority of these are seen in emergency departments or urgent care centers without requiring hospital admission, with almost 2.5 million people hospitalized and surviving to discharge. Many of these nonfatal injuries have far-reaching consequences with potential for reduced quality of life and high costs accrued to the health care system, employers, and society. In 2010, the estimated total lifetime costs associated with both fatal and nonfatal injuries occurring in any one year amount to over $420 billion (Tables 2-3 and 2-4).^3,9,10