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Keeping the Cabin Clean

February 24, 2021.

When I was very little, everybody smoked and smoking was allowed when travelling by air. My Father was a smoker and I was old enough to realise that his cigarette smoke (along with that of everybody else) would hang in the air while they puffed away - except on an airliner. Image - Alex Beltyukov

My dad and I flew quite a lot back then and I used to watch fascinated as he lit up after take-off and wondered what happened to the exhaled smoke as it never remained above me in a cloud like it did anywhere else. Where did it go?

The answer is actually rather simple and it goes back to the early days of air travel. Early aircraft did not have pressurised cabins so could only fly at a height that allowed people to breathe. Flyers learned that the thin air at high altitudes could not sustain them, and if they stayed too long above some 15,000 feet without extra oxygen to breathe, they’d lose consciousness.

Face masks to deliver pure oxygen were helpful up to a point, but in September 1934, US aviation pioneer Wiley Post demonstrated that a full-pressure suit similar to those worn by astronauts today would enable pilots to fly at heights well beyond the altitude at which airliners cruised. At the time, air travel was growing with remarkable rapidity but airline passengers could hardly be expected to don bulky pressure suits, and military flight crew would be hampered by their bulk. The only answer was to provide the equivalent of a low-altitude environment inside the cabin so passengers and crew could fly comfortably.

Without oxygen we aren’t going to survive very long, regardless of where we might be – whether it’s at sea level sunning ourselves on a beach somewhere, or at 39,000 feet going from London to New York, Frankfurt to Sydney or Tokyo to Moscow. Along with oxygen, we also breathe in whatever else might be floating around, including viruses. This is why our bodies have immune systems, which by and large, are very good at batting away such things – but what does that system do with them?

Our immune system works by analysing what we ingest and realising what is harmful and what isn’t. To make that decision means we need to be exposed to the harmful material, including those viruses; the body then builds up a natural immunity to them. It isn’t perfect and we can be (and often are) overcome, at least to a degree, so we fall ill. When we do, again the body fights whatever ails us and we recover - most of the time.

It is that harmful material that is too powerful for us to resist that kills. But if we weren’t reasonably good at resisting, the human species would have died out a very long time ago. We still however, are taking in to our bodies an enormous amount of toxic matter on a daily basis and we still get rid of it, firstly by the liquids and solids we deposit in the toilet, and secondly by breathing out.

Many people think that we breathe out only carbon dioxide but in fact exhaled air can consist of 78 percent nitrogen, 16 percent oxygen, just 4 percent carbon dioxide and potentially thousands of other compounds. Scientists suggest that exhaled air contains as many as 3,500 compounds, most of which are in microscopic amounts. Air quality can affect the content of what humans breathe in and breathe out, and some doctors suggest that monitoring the chemical content of air a human exhales can be a useful diagnostic tool in catching respiratory ailments. Human beings also exhale water vapour, a by-product of cellular respiration, at a rate that varies depending on the person, their health and other factors.

Above - Ken Iwelumo

Coming back to air travel, by the 1950s, most airliners were pressurised, and once jet engines revolutionised aircraft performance, the ensuing ultra-high altitudes at which jets routinely operate absolutely demand adequate pressure in the cabin - but from where to get the air and the oxygen? The answer is outside the aircraft (even at a great height) and via the engines’ compressor stages, where incoming air was squeezed prior to being mixed with fuel. In modern jets, warm ‘bleed air’ drawn from the compressor is cooled then ducted into the cabin.

So there we are, comfortable inside our airliner, with oxygen to breathe in and an atmosphere that is at least close to what we have on the ground. Despite it being below freezing outside the aircraft, the temperature is equable for most and we can relax in our shirtsleeves – and breathe out. We still need to know where what we breathe out goes however. Think about it for a moment; even on a short one-hour flight, the amount of toxicity in exhaled air from the inhabitants of a sealed airliner cabin is going to be huge - on a long flight, even more so and regardless of the size of the aircraft. So the air we breathe out (and everything contained in it) has to be got rid of. If it isn’t, even on those shorter flights everybody would be dead from carbon dioxide poisoning. Plant life might well like it and in return provide us with the oxygen we need, but it’s pretty bad for humans.

Obviously therefore, toxic air can’t stay inside an airliner cabin so although some is recycled and recirculated, most of it gets expelled once the doors are shut and the aircraft is on the move. On average, the air inside an airliner cabin is changed every three minutes and it has been for decades. That’s why I could never see my Father’s cigarette smoke hanging in the air – it was chucked out along with all that bad breath.

Most of today’s airliners are equipped with HEPA (High Efficiency Particulate Air) filters. That means that, on HEPA-equipped aircraft, the airflow in the cabin is similar to that of a hospital operating room with no or minimal crossover of air streams. Air is pumped from the ceiling into the cabin at a speed of about a yard per second and sucked out again below the window seats. About 40 percent of a cabin’s air gets filtered through the HEPA system; the remaining 60 percent is fresh and piped in from outside the aircraft. HEPA filters can capture very small particles, including the majority of viruses and bacteria. The outside air is naturally sterile, because of the atmospheric conditions at altitude.

HEPA filters are more efficient than the older versions used back in the days when people smoked. Even so, the old ones still worked although it didn’t stop the fast-growing anti-smoking movement from having the final word on the habit when flying. In 1988, airlines based in the United States banned smoking on domestic flights of less than two hours, which was then extended to domestic flights of less than six hours in February 1990.

Left - Kashif Mardani

That same year Air Canada adopted a non-smoking policy on all its routes, becoming the first major airline to do so, at the time describing the move as ‘a breath of fresh air’. Over the next few years, no smoking became standard across the air transport industry. It was nothing more however than a supposedly popular move that reflected the desires of those who shouted the loudest, the anti-smoking movement. No smoking when flying was hailed as a great move for healthy living, which included positive commentary from airlines themselves. Air transport carriers are of course very aware of what is perceived to be good publicity so were quick to jump on the bandwagon but the truth as far as aircraft were concerned had little to do with the health of passengers. With smoking now being banned across the industry, the air filters on board needed less maintenance; it was a money-saving move on the part of airlines and nothing more.

The fact remains that airliner cabins are remarkably clean places and always have been. They are so even more today with the onset of COVID-19; more cleaning, more frequent attention in between flights is undoubtedly a good thing and you are less likely to contract a fatal disease whilst flying than anywhere else. Nevertheless, it is not risk-free.

But then neither is anything else.

© Kevan James 2021.

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