Air Safety

Now those bans, if imposed, could be pretty upsetting to us smokers! We all know that the old durry smoke gets in your eyes, tints fingers and teeth, burns holes in the coffee cup and seat cushion, cruds up the windscreen and makes us stink to high heaven. But this allegation of affecting our flying performance is just a bit too much. So what’s it all about?

The military bible on flying, AP3456E, doesn’t say anything about no smoking in the air. It tells us that carbon monoxide (CO) from the smoke has a great affinity for haemoglobin (Hb), the oxygen carrying constituent of blood and that Hb when linked with CO, is not available for carrying oxygen and thus the effects of hypoxia will be aggravated. So what’s the problem? I never feel like a smoke when I’m hypoxic anyway.

A medical researcher, J W Heim says, ”The deleterious effects of CO on the human organism are the result of a dual action of this gas on the blood. In the first place it combines with haemoglobin forming carboxyhaemoglobin (COHb), to the exclusion of oxygen and this reduces the uptake of this gas by the blood. In the second place, it hinders the use of that oxygen which does succeed in getting into the blood by interfering with its liberation from oxyhaemoglobin (02Hb). Both phenomena act additively to produce the same result – that of oxygen lack in the tissues. ” Heim also observed that human haemoglobin has an affinity for CO, 21O times greater than for oxygen. He showed that a concentration of 0. 01% CO in the air at 10,000 feet reduces the oxygen carrying capacity of the haemoglobin by 10. 5%, thereby bringing the blood into a state of hypoxia.

Heim was mainly concerned with the dangers of allowing any CO from an engine exhaust system to enter the aircraft cockpit. He concluded that, “Concentrations of CO which are innocuous at sea level become dangerous at even moderate altitudes and it is imperative that not a trace of CO should be permitted to exist in aircraft compartments”.

Combined Effects of CO and Altitude

Another study of the combined effects of CO and altitude on visual thresholds was considered of practical importance in aviation, since flying personnel may, “through inhalation of exhaust fumes or through excessive smoking, pick up appreciable amounts of CO in their blood”. The researchers held that a pilot flying at an actual altitude of, say, 10,000 feet with appreciable COHb in his blood should be expected to perform only as well as if he were flying at 10,000 + x feet with no COHb in his blood. Thus, with COHb in his blood he may be regarded as flying at a ‘physiological altitude’ of 10,000 + x feet as contrasted with an actual altitude of 10,000 feet.

To obtain reliable quantitative data, the researchers conducted laboratory tests relating to visual efficiency. Although this is only one of the activities of the pilot, it is certainly an important one. Moreover, the retina is essentially part of the central nervous system and visual changes may therefore reflect similar alterations in other functions of the central nervous system.

Firstly, the researchers demonstrated that the effects of CO hypoxia and hypoxic hypoxia are practically equivalent. The effects of a given percentage of COHb is roughly the same as that of an equal loss of percentage 02Hb due to high altitude. Secondly, they determined, as regards visual intensity discrimination, a quantitative basis for calculating the effects of CO in the blood on the pilots’ ‘physiological altitude’.

Their research showed, for example, that if a pilot is flying at 6,000 feet with 0. 005% CO in his air, his physiological altitude is increased to about 12,000 feet. The same amount of CO at 9,000 feet raises the physiological altitude to almost 15,000 feet, and so on. Thus the pilots’ ‘physiological altitude’ is considerably increased over the true altitude at which he is flying. These figures assume that the airmen are exposed to this amount of CO for at least several hours.

The visual tests employed in this study proved to be so sensitive that even the effects of the small quantities of CO absorbed from cigarette smoke were clearly demonstrated in a few preliminary experiments. Deep inhalation of the smoke from a single cigarette, causes an increase in the COHb saturation of almost 2%. Although prior to these tests such a small amount was considered insignificant, it nevertheless caused a distinct impairment of visual sensitivity. After three cigarettes, the blood COHb was 4% and the effect on one subject’s visual sensitivity was equal to that at an altitude of almost 8,000 feet.

The researchers concluded that, “The amounts of CO allowable in the cockpit in flight should always by considered in terms of the accentuation of the effects of altitude. The importance of refraining from excessive smoking, particularly before a flight, is also made obvious”.

Symptoms of Hypoxia

To get this ‘physiological altitude’ effect into perspective, let’s go over the sequence of onset of symptoms and decrement in performance from hypoxia with an increase in actual altitude:

• 0-5,000 feet; ideal, asymptomatic.

• 5,000-8,000 feet; no symptoms in resting healthy person except for decreased night vision.

• 8,000-10,000 feet; progressive cerebral deterioration frequently with insidious onset, headaches, visual changes, defective judgement, poor discrimination slowing of reaction time, and exhilaration.

• 10,000-20,000 feet; extension of above with weakness, cyanosis of nails; tremors of hands, fingers and head, time of usable consciousness – 10-20 minutes.

So for a pilot in flight at an actual altitude of, say, 6,000 feet, only a slight increase in ‘physiological altitude’ will cause the onset of symptoms of hypoxia and consequent decrease in performance.

Visual Problems at High Speed

Vision is one attribute that is soon affected by the onset of hypoxia. Vision is more than just the act of seeing, it depends also on the proper utilisation of the eyes and the correct interpretation of the visual picture by the brain. Every good knucklehead knows that as the speed of flight increases, it is no longer valid to think in terms of an instantaneous visual picture. Perception time becomes significant and consists of:

• the transmission time from the eye to the brain after an object has been perceived on the edge of the field of vision;

• the time for the eyeball to move and look at the unknown object; and

• the time involved in recognising the object which is now Sharply ‘pictured’ in the brain.

This overall time delay, from visual pickup through ‘lock on’ and recognition, is of the order of 1-2 seconds, Perception time is further increased if the visual or mental response of the pilot is impaired for any reason such as debility, the effects of G, or hypoxia. The delay can represent a considerable closing distance and an object, suddenly appearing, may not elicit any useful response as the pilot could be past the object before he saw it and reacted.

Night Vision

Despite all the electronic gadgetry assisting the pilot of a modern aircraft, night vision continues to play an important role in military flying. The rods of the retina – the receptors responsible for night vision – are highly sensitive to oxygen deficiency. A deterioration in night vision is perceptible with even minimal decrease in the partial pressure of atmospheric oxygen.

In 1970, H A Pretorius conducted quantitative tests on a large group of healthy individuals to assess the effects of hypoxia on night vision. Threshold curves of dark adaptation, with and without the administration of oxygen, were obtained at a height of 5,000 feet. Following the administration of oxygen, an average improvement in night vision of 25. 9% was found.

Pretorius commented on his findings that, “The improvement of 25% at the lower levels of dark adaptation doesn’t mean that a person will see especially well in the twilight or dark and would not make much difference to a person who goes for a stroll after dinner. During flight however, it does have some significance. If a pilot is approaching the ground at 150mph in twilight or dark, with clouds lying low over the runway, a crosswind blowing and rain further hampering vision, then the pilot sometimes requires super normal perceptual abilities”. Pretorius concluded by stressing that the administration of oxygen is necessary during all night flights.

Night Adaptation

Of equal importance to aircrew is the adaptation time for night vision. The rods are able to perceive an unlit object at night by means of a substance known as visual purple. If this substance is broken down or bleached by bright light, it takes about 30 minutes to rebuild it before maximum sensitivity is acquired. In contrast to this 30 minute adaptation period, it takes only one second for the effect to be lost if the eye is exposed to bright light. Thus the very action of lighting a cigarette effectively destroys night vision for a significant period.

Summary

Okay! Are you starting to see the big picture? Let’s just look over what facts we have on smoking and flying:

The average concentration of CO in cigarette smoke is about 2%, or about 0. 04% in the inhaled mixture of smoke and air. Human haemoglobin has an affinity for CO about 210 times greater than for oxygen. It takes about 24-36 hours for all traces of CO to be removed from the blood after exposure to CO. • The effects of CO hypoxia and hypoxic hypoxia are practically equivalent. For example, a pilot at 6,000 feet actual altitude inhal- ing air containing an average of . 005% CO for a three hour period, would have a ‘physiological altitude’ of 12,000 feet. • The CO absorbed by the blood when three cigarettes are smoked at sea level can produce a ‘physiological altitude’ of 8,000 feet.

• Perception time is increased when mental response is impaired by hypoxia.

• Deterioration of night vision is apparent with even a limited degree of hypoxia. From a ‘physiological altitude of 5,000 feet, a significant (25%) improvement in night vision occurs with the administration of oxygen.

• It takes about 30 minutes for the eye to adapt to night vision and about one second to lose it.

Well that’s about all then. The poor old cigarette smoker is behind the eight-ball right from the start. If you smoke you have CO in your blood, which in itself is a deadly gas, and that stops a lot of oxygen from getting to where it is really needed. Without an adequate supply of oxygen the body shuts down those facilities we most need in flying-good vision, accurate discrimination and sound judgement are impaired with consequent effects on our performance.

If we are serious about flight safety and professionalism in the air, we have to make smoking a dying habit in the RAAF and observe those ‘No Smoking Within 30 Metres of the Aircraft’ signs at all times. If we are not prepared to go that far, we should be constantly aware that smoking does affect our performance, especially the difficult to appreciate facilities like judgement and we should make the necessary allowances for our degraded performance.