Firefighting is one of the most physically demanding jobs imaginable. In addition to physical exertion, a firefighter must also wear breathing protection while doing the job. How does a respirator affect the fire professional?
Flames. Heat. Water spray, humidity. Falling debris, wind changes, embers. Smoke and toxic fumes. The professional firefighter relies not only on clothing, but often also on respiratory protection. That’s where breathing physiology comes in.
Breathing physiology might sound daunting, but any fire professional can benefit from knowing the basics.
There are many factors that can have significant effects on our breathing, including:
- Oxygen levels in the air
- Work rate (rest, steady work, sudden force etc.)
- Emotions (fear, worry)
- Restrictions (respirator, lung disease, smoke inhalation)
- Speech (talking, coughing, sneezing)
These factors influence every breath: how fast or slow, how deep or shallow, how abrupt or smooth. Don a respirator, and the effects become even more apparent.
We breathe without thinking. We take a breath around 14 times per minute at rest without even noticing. But plenty goes on in the background:
- The respiratory system draws fresh air into the lungs, where oxygen enters the blood stream. ‘Spent’ air is exhaled.
- A normal breath at rest is about half a litre. But the greatest volume of air that can be held in the lungs is around ten times that in many adults. Some athletes, wind-instrument musicians (and glass-blowers!) train their lungs to even greater capacity.
- The difference between inhaled and exhaled air is not great. Our lungs absorb only about 4% of the oxygen in the air. Exhaled air normally contains about 4% carbon dioxide, as well as more water than the air we breathe in.
- We spend roughly one-third of the time breathing in during rest. Exhalation comprises around 2/3 of each breath.
- It’s carbon dioxide, not oxygen, in our blood that governs much of our breathing. There are no oxygen sensors in the brain – only CO2 sensors.
- The heart pumps around 5 litres of blood per minute at rest. This can rise to 35 litres at hard work, when the entire volume of blood in our body runs through the heart 7 times every minute.
Breathing and the firefighter
The fire professional spends a lot of time at high work rates. During hard work, the muscles need up to 100 times more oxygen than at rest, and the heart must work 8—10 times harder.
Physical work brings on many changes in the body, including:
- Increased pulse
- More blood per heartbeat
- Faster breathing
- Higher body temperature
- Higher blood pressure
- More blood to the muscles
- Greater lung absorption to maximise use of red blood cells
A hot topic
Heat is one of the firefighter’s concerns, not just because of the physical hazards associated with fire, but the physiological effects as well.
Increased ambient temperature greatly affects the body. Add to that the need to wear impermeable clothing.
It has been shown that the human body can ‘learn’ to work more efficiently in heat. Research has found that a veteran firefighter might have normal pulse and body temperature even after two hours in a +50°C environment. By comparison, a novice subjected to the same conditions might have a racing pulse and high fever.
However, recent Australian field studies confirm that even ‘old hands’ can experience a core body temperature rise of as much as 2.5 degrees in just the first 15 minutes of an emergency.
Another important factor to the fire professional is humidity, from both the weather and from the copious amounts of water used.
Humidity has a significant effect on body temperature. The body’s main way to keep cool is perspiration. The evaporation of sweat is crucial: in the first half hour of the job, the average firefighter might lose as much as 1.2 kg of fluid by way of perspiration. Thick turnout clothing doesn’t help.
…and now put on a respirator!
Respirator wear complicates things. Regardless of type, brand or model, the mask adds to the workload. This is called Work of Breathing (WoB) and means the extra work the lungs must perform because of the respirator. At first, this may not seem even noticeable to the wearer, but can affect work performance considerably over time. It is important to select a respirator with as low breathing resistance as possible.
The most common respirator type used in firefighting is positive-pressure self-contained breathing apparatus, which adds even more to the workload by dint of the weight of the backpack. It is therefore essential to make sure that the cylinders are as light as possible (such as carbon fibre), and are carried on an ergonomically designed harness.
You never completely empty your lungs: this is a physiological impossibility. Some ‘spent’ air containing carbon dioxide will always remain in your lungs and upper respiratory tract. This is called dead-space. Putting on a respirator will further add to the dead-space.
Regardless of what type of respirator is used, a vicious circle can occur:
- Increased dead-space + greater workload ‘ more CO2 in the blood
- More CO2 in the blood ‘ quicker breathing
- Quicker breathing ‘ shallower breathing
- Quick and shallow breathing ‘ even more CO2
- More CO2 ‘ psychological stress
- Psychological stress ‘ even quicker breathing
In many cases, the vicious circle can be broken by consciously taking deeper, slower breaths.
Inexperienced BA users are particularly at risk. Remembering the limited supply of air, it may be easy to think that you can save air by taking small breaths. This is false economy, and raises the risk of entering the vicious circle. A person taking deep breaths makes much better use of the available oxygen.
This is the paradox of breathing from a cylinder: deeper breaths mean lower air consumption.
Go with the flow
The harder you work, the more air your body needs. The more air you need, the faster it must travel to the lungs.
It is important to understand the difference between air volume and air flow. This can be a little tricky, as both are expressed in litres per minute (l/min). ‘Volume’ means how much air you breathe every minute, while ‘flow’ means how fast the air travels at any moment.
Without getting too technical, the limitations of a respirator are greatly dependent on the air flow. In short, if your requirements for air are greater than the respirator can deliver, you will out-breathe the equipment, and smoke may leak into the mask and reach your lungs.
How fast can you go?
At rest, the human body needs about 5—8 litres of air every minute. A leisurely walk requires perhaps 10—25 litres. At jogging pace, you might consume up to 50 litres, while hard work may demand up to 100 litres of air per minute.
But that doesn’t mean that the air travels at 100 l/min to your lungs. Why? Because in the breathing cycle, inhalation is only one-third of each breath (in real life a bit more, but we will keep things simple). In short, 20 seconds of every minute is spent on breathing in.
To use the simplified maths above, for a person to take in 100 litres in just 20 seconds, the air must move at a speed of 300 l/min through the mouth (and through the respirator).
But don’t forget that each inhalation begins and ends at zero air flow (at the ‘bottom’ and ‘top’ of every breath, when the lungs are still). At the start of each breath, the air speeds up, then reaches maximum speed, and then slows down to zero again. The maximum speed is called Peak Inhalation Air Flow, or PIAF. This peak flow is very fast indeed. A firefighter can frequently reach peak air flows of 400 l/min, and sometimes even faster. The respirator must be able to cope with such air speeds.
Speak to me
Another factor that has a great influence on your breathing is speech. Speech renders the breathing pattern much more erratic: sudden spurts, sharp spikes, placing very high demands on the respirator.
Good speech transmission is important. A respirator with poor speech characteristics or – even worse – with high breathing resistance will only tempt the wearer to take off the mask, either to speak, or, paradoxically, to breathe. And only a few seconds without a mask can ruin several hours’ breathing protection, with immediate dire consequences if the air is toxic.
What to look for in a respirator
When selecting a respirator of any type, the following list may be of use. Some of the points are especially applicable to firefighters:
- Minimal inhalation resistance
- Minimal exhalation resistance
- Minimal dead-space
- Light-weight cylinder material (BA sets)
- Light-weight harness (BA sets)
- Ergonomic design of harness (BA sets)
- Best possible speech transmission (electronic if available)
- Compatibility with other safety gear
For more information, go to www.sea.com.au