Do wildland firefighting safety zone separation distances need to be increased to reduce the risk of injury or death? A team of researchers recently examined this issue to see if current models provided the best recommendations. Study results indicate that firefighters may want to consider slight increases in separation distances.
Safe separation distance models
Wildland firefighters operate in a difficult environment where a variety of factors including topography, vegetation and quickly changing weather and fire conditions can lead to great risk of injury or death from burnovers or entrapment.
There are various models that provide firefighters with estimates of safe separation distance: the distance between themselves and the flames necessary to reduce the risk of burn injury. One widely known model from a 1998 study suggests that firefighters should have a separation distance of at least four times the height of the flames. However, this and other physically-based models for estimating safe separation distance have known limitations such as using only radiant heat transfer, no slope influence, no inclusion of injury mechanisms apart from burns to bare skin, and no field validation.
A recent study1 was the first to create an empirical model using actual field data reported in wildland firefighter entrapments from 1935 to 2015. It indirectly took into account both convective as well as radiant energy heat transfer. (See image, right)
How heat is transferred
Image credit: U.S. Nuclear Regulatory Commission
What was learned
This was the first study to confirm that the use of fire shelters significantly lowers the likelihood of fatal and non-fatal injuries during an entrapment situation. The empirical data also support the belief that vehicles can be used as places of refuge or as shields.
It confirms that slopes limit the effectiveness of fire shelters to the extent that each percent increase in slope steepness increases the odds of a fatal injury by 3 percent without the aid of a fire shelter.
The study proposes a safety zone separation distance that accommodates not only radiant energy transfer but also the convection of hot combustion gases and excessive smoke exposure.2 The empirical data from past entrapments would suggest that safety zone separation distances should be larger than the distances currently recommended by the physically based models, for example “4 x the flame height.”
Brush fuel types were the most dangerous in terms of causing non-fatal injuries.
Learn more about this research
The research article is available through our library by contacting FEMA-NETCLibrary@fema.dhs.gov. Interested readers may be able to access the article through their local library or through the publisher’s website.
1Page, W. G., & Butler, B. W. (2017). An empirically based approach to defining wildland firefighter safety and survival zone separation distances. International Journal of Wildland Fire, 26(8), 655. doi:10.1071/wf16213
2The risk tolerance thresholds for this study are very conservative and may over predict what may be needed if conditions are less severe.
Estimating safety zones
This National Interagency Fire Center video from 2015 shows how to calculate the increase in safety zone sizes when considering slope and wind.