Generally, under the U.S. Constitution, police cannot make an arrest or seize property without a warrant or unless you give your consent.

However, where exceptional or dangerous circumstances exist, peace officers are permitted to seize or detain people and evidence without a warrant. These are known as exigent circumstances.

Threats & Emergencies
The simplest definition of exigent circumstances is emergency situations. If there is an emergency situation, police do not need a warrant to arrest you or to search your property. Emergency situations, in fact, make the arrest reasonable. Some emergency situations that qualify as exigent circumstances are:

• Threat of disappearance – The police pursue a fleeing suspect into a house or apartment to make a warrantless arrest, believing the suspect is inside and may escape.
• Threat of destroyed evidence – Police see that evidence might be destroyed in a fire or flood, or where the evidence will disappear with a fleeing suspect.
• Threat to safety of public – The police enter your home if you put the public in danger, or if you are in need of immediate attention. Additionally, if the police approach your house and see you reach for something, they may be justified in entering out of concern for their own safety. But only the initial entry is justified; the police may not make a warrantless search of the entire house without more justification.
• Threat to property – The police may enter a building or home to control a fire, to search for bombs, and to look for a drug lab after smelling strange odors.

Factors that Contribute
Whether exigent circumstances exist will depend on the facts of each particular case. Some of the factors that courts use to determine exigency are:

• The seriousness of the offense – The more serious and dangerous the offense, the more likely that the police are justified.
• Whether you are armed – If the police have a reasonable belief that you are armed, they are more likely to be justified in arresting or searching you.
• Whether probable cause is clear – The more clear, the more justified the police are.
• Whether you are likely to be found at the location – Police cannot search for you if they do not have a reasonable belief that you will be found at the location.
• Likelihood of escape – If you are likely to flee or escape, the police may be justified in pursuing you.