In the United States, the training standards and scope of practice of Emergency Medical Technicians (EMTs) are set by the National Highway and Traffic Safety Administration (NHTSA). New EMTs are taught the basics from the approved curriculum and are subject to both cumulative knowledge and skills assessments. Throughout their instruction, EMTs learn a variety of skills, but scene safety is a topic that permeates every lesson taught. During the beginning of their practical skills assessments, one of the first questions an EMT must learn to ask their instructor is, "Is the scene safe?" 
Issues of Concern
Scene safety is paramount in the Emergency Medical Serivice (EMS) profession. When EMS providers arrive at the address of an emergency, they are entering an unknown situation that could be unpredictable. If a scene is considered "safe," this means there are not any obvious potential threats or hazards that could impede the EMTs from doing their work or bring them harm. This could be anything from an electrical hazard to an active shooter to an agitated patient. When entering a scene, EMTs are taught priorities in a specific order: 1) personal safety, 2) safety of their partners or colleagues, and 3) care of the patient. Any threat to one prevents moving forward to the next. 
It is also important to note that just because a scene is initially "safe" does not guarantee that it will remain so. Perhaps a new individual arrives and poses a threat. Maybe a fire starts. Regardless of the source of the new hazard, scene safety is continuously evolving and requires constant reassessment.
There are many potential hazards that first responders may encounter on the job.
A proper assessment of scene safety begins at the time of dispatch. The call-taker has collected information from the 911 call, and this can prove invaluable. The description of the emergency and the destination address are critical pieces of information and are useful for preparation. In the same way that notification of a cardiac arrest has responders preparing for CPR, notification of a motor vehicle accident should trigger thoughts about having police on scene for traffic control or dispatch to a chemical plant should cause consideration of potential hazards. 
Advance knowledge of the neighborhood can also be helpful. Many full-time first responders are expected to be familiar with the details of the territory within their service area. Awareness of areas known for high rates of criminal activity can be useful in safety preparation. If responders are aware that their destination is an active crime scene, law enforcement should secure the premises before EMS entry. 
Observation is a valuable tool when entering an unknown situation. Noting the actions of bystanders near the scene can be suggestive. For example, someone fleeing the scene on EMS arrival may suggest foul play. Large crowds gathering could be dangerous to the safety of the first responders -- especially if the crowd appears agitated. Noting the number of cars parked on the street or in the driveway could indicate the number of people inside. Broken windows or doors should raise concerns. 
Responders should always approach the scene with caution, and any suspicious activity merits serious consideration. It is often suggested that first responders stand to the side of the doorframe when knocking to minimize themselves as a potential target. First responders should be vigilant about their safety and prepared to act at all times. It is essential to have an exit strategy in mind. When there are multiple bystanders on a scene, the location of these people is crucial information. If at all possible, they should not be allowed to get behind first responders, minimizing the risk of an unseen attack from behind. Bystanders should also not be allowed to block exits. First responders should have an exit strategy in mind for themselves if necessary. 
Close monitoring of bystanders is also important for another reason. Family members or other people in the house may be disruptive verbally and physically to patient care. Recognition of body language and any concealed possessions (e.g., potential weapons) may head off potential threats. For example, an agitated or anxious person may have their arms crossed or be pacing the room.
There may also be animals involved at the scene. Large, agile animals may pose a particular threat. Growling, barking, and foaming at the mouth can be signals of animal agitation and impending attack. First responders should avoid sudden movements and retreat to a safe distance. 
Drugs and drug paraphernalia are other hazards often encountered. EMTs should attempt to familiarize themselves with the appearance of common substances in their areas. Especially with unidentified substances, caution is necessary; and the first responders should always be mindful of the potential irritant or explosive properties of these agents. Caution should be exercised as there may be exposed needles or other sharps in this situation. 
Hazardous materials pose their own set of challenges. If possible, EMTs should avoid entering a hazmat scene entirely. The ambulance should ideally be parked up-wind, a long distance away from the scene, and the appropriate authorities should be notified. 
EMS providers may also find themselves in what are typically considered "extreme environments" based on where they are practicing. For example, in a mountainous area, more technical rescue skills may be required depending on the demands of the situation which inherently increases the risk. 
The above is not an exhaustive list of all possible threats to the safety of first responders. Each individual should be cognizant of their situation at all times. The goal of each operation should be safe extraction and delivery of the patient to the desired destination.
In the prehospital environment, first responders are exposed to a more fluid situation than their colleagues within the hospital. They must be vigilant about their own safety. This vigilance takes instruction and practice but is necessary to ensure they will be healthy and able to provide care during emergencies. 
Scene safety is an important concept to remember and practice when working in the field. First responders should always protect themselves and their teams while working in the field -- both before and while rendering patient care. Keeping a high suspicion for danger through observation and preparedness will result in a successful mission. First responders must be safe and healthy to ensure that their patients remain safe.