With the destructive power of a fire, walls, structural supports and other parts of the structure may become damaged and weakened and can collapse with activity. Area first responders explained why it is important for homeowners to remain cautious when reentering a home following a fire as some invisible dangers can linger long after the last embers have been put out.

While a homeowner may want to reenter a home to gather belongings following a fire, officials said it could take some time before fire officials can clear a site for reentry.

“The biggest threat is going to be structural stability,” Grayson County Fire Marshall John Weda said Friday. “It take cake some time to shore up a building and make sure it is safe.”

Following a fire, it is common for a structural engineer to come in to determine if a structure is safe for reentry, Weda said.

Modern homes typically make heavy use of truss construction to allow boards to be placed quickly, but this construction can collapse more easily than older techniques when exposed to high temperatures.

Dustin Elk currently works as an emergency nurse practitioner with Wilson N. Jones Regional Medical Center, but previously served as a paramedic with Sherman Fire-Rescue.

“Whenever a structure is brought to an increased temperature, it is not stable any more,” he said.

Even if a building is structurally sound, it is common for police or fire personnel to be on site as a resident picks up some belongings as an additional protection.

While a weakened structure is a visible threat, other hazards are invisible to the naked eye. As things burn during a house fire, many release unseen chemicals in the air that can continue to be generated even after the flames have been extinguished, Weda said.

The most common of these chemicals is carbon monoxide, an odorless chemical which can be released through the combustion of many household items. Hydrogen cyanide is another chemical that is typically released when furniture burns, Weda said.

As another example, Elk said common household refrigerants can release a chemical that is lethal in humans at a density of about two parts per million in the air.

Another airborne threat in older structures is possible exposure to asbestos fibers. The material was used as a common insulation product throughout the first half of the 20th century that was later determined to be a carcinogen. While it is typically harmless when in place, its destruction or removal can create tiny fibers that can be inhaled.

The Mesothelioma Center said these fibers typically don’t pose an immediate threat following a fire as the water from fire fighting efforts suppresses the dust. However, once a site is dried these fibers can be kicked back into the air.

In the event that someone must enter a contaminated site, the group recommends the use of a half-face respirator approved by the National Institute for Occupational Safety and Health.

Weda said fire fighters and first responders are equipped will often use sensors to detect these and other chemicals in the air following a fire. In many cases, it can take a few hours for these chemicals to dissipate into the air, but this can vary depending on how enclosed an area is.

Other threats include chemicals that can be harmful when mixed with water, but Elk said these are not common in a household environment. However, they can be a threat in a more industrial environment.