Risk analysts divide risk of potential negative events into two parts: incidence and consequence. Incidence is how often the event happens or how likely it is to happen in a given place in the next month or year. Consequence is how bad the event would be. Some events may be very unlikely but very bad, and measures to reduce those risks or put management plans in to respond to the event are merited. Other events are common but not so bad. At a facility that generates or stores medical waste many combinations of probability (incidence) and consequence (severity) are possible, but usually the consequences are not catastrophic.
Despite the plots of some science fiction stories, it is unlikely that a release of medical waste would yield an epidemic or large-scale casualties. It is theoretically possible but there are no reports of it ever happening. Further, a “dirty bomb” with medical waste instead of radioactive waste is considered unlikely to be effective by experts.
Pathogens in medical waste pose a disease risk, and if the disease is contagious (or if the waste is released over a large area), a potential public health problem. The reason we treat infectious waste before disposal is to substantially reduce (for all practical purposes eliminating it) risk of disease transmission. Diseases caused by improper disposal of waste include parasitic infection, lung infection, Bacterial Meningitis, HIV, Hepatitis B, Hepatitis C, fungal infections, and Bacteremia (bacteria in the bloodstream).
The EPA has said that, in general, risks posed by medical waste are greater for health care workers than for the general public. This is because the maximum disease-causing risk is at the point of generation and over time diminishes.
The US Centers for Disease Control and Prevention states "no epidemiologic evidence suggests that traditional waste-disposal practices of health-care facilities (whereby clinical and microbiological wastes were decontaminated on site before leaving the facility) have caused disease in either the health-care setting or the general community."
Medical waste can also include non-infectious hazardous components. These might be materials that are deemed hazardous by their ignitability, corrosivity, reactivity, or toxicity to living organisms. Hospitals produce radioactive waste because radioactive materials are used in both diagnosis and therapy. Engineers have ways of treating waste to reduce those risks, too.
Experts say the most risky type of medical waste as far as transmitting diseases is microbiological wastes such as you might find at a laboratory. Sharps (needles, etc.) also pose a risk for injury.
In the writing of permits and of operations plans, engineers might conduct a risk assessment that includes a projection or prediction of waste risks at points in the facility operation and waste handling process. If the volume or composition of medical waste being generated varies, this risk assessment might be questionable, but it is often the only thing operators and regulators have.