Impact of Heat Radiation on People and Siting your bushfire shelter

A lot of attention is paid to a building’s ability to survive a bushfire with the BAL rating process, however there is little discussion on the impact of this level of heat on people. Understanding this helps inform what to do when a bushfire threatens and how to site a bushfire shelter.

In the offshore oil and gas industry (my working career), we look at risk to personnel as a result of thermal radiation from a fire, since that is one of the primary risks in the business. There’s lots of different numbers and impacts thrown around, because unsurprisingly the results tend to be calculated, and are conservative.

The information below shows that people can not withstand even the lowest level of design heat flux. Staying in your house is hazardous even if it has a BAL rating, and fleeing it will not be possible. For a full blown bushfire, you will not be able to stay outside and fight the fire, even ignoring the smoke, when it gets close.

Bushfire management plans are essential, with the leave early advice well justified.

We will have a bushfire shelter installed, and it needs to be located in the safest location possible. Our considerations, as an example follow.

In Victoria, it’s usually the north winds that bring the worst fire storms, and our “worst” bushfire vegetation is also located to the north and north west. Our shelter is therefore located to the south of the house so that it can provide heat radiation protection as we evacuate to the shelter. Solid objects are very effective at blocking the radiation, so we should be safe to run away, with time to get into the bunker.

It’s on the south east side to be as far away from the bush as possible – the dam is to the east, so this means we have the maximum distance from the bush to the west. The main road is also to the west – a likely fire direction due to someone throwing out a cigarette etc on the road While land slopes slightly down to the south, and fire travel faster uphill, this is the least likely fire direction, and still has a large area of exposed gravel (from the mining 30 years ago) just behind the tree line, so a fire won’t be able to take advantage of the slope. We will also have our waste water dispersal field between the shelter and the bush to the south, so the area should always be fairly damp.

We will wear appropriate clothing to shield us from the heat radiation, such as:

  • Long-sleeved shirt and pants made from a natural fibre such as cotton or wool.
  • Sturdy boots and woollen socks.
  • Tough leather gloves.
  • A wide-brimmed hat.
  • A face mask or towel to cover your mouth and nose.
  • Eye protection such as smoke goggles. Offshore, we use smoke hoods to get to our emergency escape boats. I’m still thinking about this one!

I’ll be wearing my flame retardant, high visibility overalls as well.

We will stop defending and leave the house in advance of the flame front to make sure we can make it into the bunker without cooking ourselves, or filling the bunker with smoke.

Heat Radiation levels:

For context, maximum solar radiation ~ 1 kW/m2.

The lowest level of housing protection from a fire is for BAL 12.5 – 12.5 kW/m2.

1.58 kW/m2 – continuous exposure allowed with appropriate clothing (API RP 521)

4.73 kW/m2 – emergency actions lasting 2 to 3 minute allowed with appropriate clothing (API RP 521)

6.31 kW/m2 – emergency actions lasting 30 seconds allowed with appropriate clothing (API RP 521)

Between 5 – 12 kW/m2, people would not voluntarily enter an area, but could escape with minor burns.

Between 12 – 37.5 kWm2, escape may be possible within a few seconds, suffering severe burns.

Above 37.5 kW m2 thermal radiation, persons will likely die in a few seconds.

The following table, from the Handbook of Chemical Hazard Analysis Procedures shows that a person would suffer 2nd degree burns on bare skin within a few seconds of being exposed to a 12kW/m2 radiation level.

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