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Carbon Dioxide in the Air






It is to Dr. Black that we owe the first proof of the existence of carbon dioxide in the air, during the years 1752-1754. He termed it fixed air. Lavoisier, however, showed that it was a compound of oxygen and carbon.

The actual proportion of carbon dioxide in the air varies very considerably according to circumstances. Whalley reported that in a Scottish mine the carbon dioxide in the air near the coal face reached 1.21 per cent., whilst on the pavement it was no less than 4.56 per cent. Lewy, in discussing the abnormal air of New Granada, points out that owing to forest fires the percentage of carbon dioxide in the air would often rise to 0.49 per cent. These cases, however, are abnormal.

The average amount of carbon dioxide in pure, fresh air may be taken as 3 parts per 10,000 by volume. This is subject, however, to alteration by a variety of factors. Thus over land it increases slightly during the night, and it is somewhat greater over land than over sea. In Antarctic regions it is less than in more temperate climes. Fog and mist likewise play an important part. Angus Smith, in his summary of the then available data, showed that the average carbon dioxide content of Manchester air in times of fog amounted to 6.79 parts per 10,000, the normal amount for Manchester being 4.03. Similarly, Russell observed as much as 14.1 parts of carbon dioxide in 10,000 of air in thick, white London fog. Indeed, his average for twenty-nine fogs was 7.2, whilst for ordinary clear London air he found only 4.0 parts. Rain is accompanied by a diminution of carbon dioxide, but with snow the gas is in excess. Its variation with altitude is apparently inappreciable, but the evidence of the effect of latitude is conflicting.

Considerable variation may be due to local circumstances. Thus Reiset found that the presence of a flock of 300 sheep on a fine, calm day in Dieppe induced a notable rise in the proportion of carbon dioxide in the immediate neighbourhood, which registered 3.18 parts per 10,000 instead of 2.96 - the normal value for Dieppe. The influence of vegetation upon the carbon dioxide content of the air has not received the consideration it deserves, although a few isolated experiments have been carried out. Ebermayer, for example, found an excess of the gas in the forest, a result that confirmed the earlier observation of Truchot. In confined spaces, such as dwellings, the carbon dioxide shows a marked increase owing to its being a product of human metabolism, and may reach 0.5 per cent.

A connection has been traced between the relative amounts of carbon dioxide and ozone in the atmosphere, the amount of the former varying inversely with that of the latter for values below the normal.

The soil is continually evolving carbon dioxide; part of this is no doubt of volcanic origin, and part is due to chemical processes of a more superficial character. As an instance of the former, it is interesting to note that after eruptions of Vesuvius the soil has breathed out such vast quantities of carbon dioxide that game have been poisoned wholesale. The famous Valley of Death in Java, the Death Gulch of Western America, the Grotto del Cane near Naples, and others, owe their poisonous atmospheres to carbon dioxide exhaled by the soil.

It has been calculated that each square mile of fertile garden soil evolves some 4000 tons of carbon dioxide during the summer months. This is due to the decay of vegetation and organic matter, as well as to the continued respiration of small creatures. Thus, for example, earth-worms breathe out as much carbon dioxide as a human being, weight for weight.

Nevertheless, the carbon dioxide content of fresh air remains fairly constant at 0.03 per cent., a fact which suggests that some reactions must be proceeding on a grand scale, tending to absorb or destroy the gas. Three such have been discovered. First there is the action of plants, the green parts of which under the influence of sunlight inhale carbon dioxide and exhale pure oxygen. As illustrating this it may be mentioned that one square metre of leaf surface of Catalpa bignonoides in full sunlight is capable of assimilating 344.8 c.c. of carbon dioxide in a single hour. When the vast amount of foliage in tropical and temperate climes is considered, it can readily be imagined that this plays no small part in the reoxygenation of the atmosphere; indeed it has been calculated that leaf action alone would suffice for the purpose. There can be no doubt that the amount and luxuriance of vegetation does respond within certain limits to the amount of carbon dioxide in the atmosphere, and existing coalfields probably represent Nature's attempt to reduce the percentage of this gas by locking it up as carbon beneath the crust of the earth.

A second important regulator of the atmospheric carbon dioxide is to be found in the rocks of the earth's crust. T. C. Chamberlin calculates that the carbon locked up in the sedimentary rocks of the earth's crust is 30,000 times as much as is now existing in the air, and he further estimates that 1620 million tons of carbon dioxide are being annually withdrawn from the air in the course of building up new sedimentary rocks.

Finally, the ocean serves as a vast regulator, as was first pointed out by Peligot in 1855. Rain-water dissolves carbon dioxide from the air, and on reaching the soil absorbs yet more, both in the free state and in combination as carbonates. The streams and rivers carry this away and discharge it into the sea. There can be no doubt that each of these factors plays an important part in regulating the composition of the air, but the actual share borne by each will vary according to circumstances.


Estimation of Atmospheric Carbon Dioxide

A convenient method is that of Pettenkofer, which consists in introducing a standard solution of barium hydroxide into a large bottle containing several litres of the air to be examined. The bottle is shaken from time to time to keep the sides moistened with the solution, and after 5 or 6 hours the absorption of carbon dioxide may be regarded as complete. The baryta solution is decanted into a small stoppered bottle and allowed to stand until any suspended barium carbonate has settled. A portion of the clear liquid is then removed and titrated with dilute sulphuric acid, using phenolphthalein as indicator. The diminution in alkalinity due to combination with carbonic acid is thus measured, and from the data obtained the percentage of carbon dioxide in the atmosphere may easily be calculated.

The results obtained are frequently irregular and invariably indicate too high a percentage of carbon dioxide in consequence of the absorption of expired air from the operator during the titration. To obtain accurate results many precautions must be observed.

Carbon dioxide may also be estimated volumetrically by absorption in concentrated potassium hydroxide solution from a volume of air. The diminution in volume is noted by direct measuring, and results of considerable accuracy can be obtained in this manner within a very few minutes.

Gravimetric methods are, in general, more accurate, but require a considerable amount of apparatus, and take a longer time to execute. The simplest method consists in slowly aspirating some 40 litres of air over caustic potash contained in U-tubes and noting the increase in weight. The air must first be dried by passage over concentrated sulphuric acid, which simultaneously removes any ammonia.

Physiological Significance of Carbon Dioxide

Carbon dioxide is a colourless, almost tasteless, and odourless gas, and it is consequently impossible by the unaided senses alone to detect its presence in the air. The odour of respired air is not due to carbon dioxide, but to other gases accompanying it, and the close, stuffy effect of ill-ventilated buildings is due to the same cause. From a physiological point of view, carbon dioxide is of as much direct importance to us as to our complementary organisms, the plants. " A certain percentage of carbon dioxide in the atmosphere is essential to our very existence from minute to minute; not only is it the normal stimulus to the respiratory centre, but it assists in the splitting up of oxyhaemoglobin in the tissues. If an animal is made to breathe a carbon dioxide free atmosphere, the normal circulating carbon dioxide in its blood is reduced, and there arises after a time a condition known as acapnia, in which the respiratory centre - missing its customary fillip - goes on strike, stops breathing, and the result may be fatal; restore, however, the due proportion of carbon dioxide and the breathing is resumed." In other words, the animal organism is so accustomed to breathe air containing traces of carbon dioxide that it cannot do without it.

It is equally true that an excess of carbon dioxide is fatal to human life, although it is difficult to determine the exact amount that is so, inasmuch as it varies with a number of factors, such as the health and individuality of the person, and the time during which the gas is inspired. In breweries and mineral-water factories large quantities of the gas are regularly inhaled by the men. Beadnell mentions that 20 per cent, of pure carbon dioxide has been inhaled for three hours without fatal results. On the other hand, carbon dioxide from a candle or gas flame is harmful at much smaller concentrations. Possibly this is due to the relatively large amount of moisture simultaneously produced, for Grandis, in a series of experiments made upon the venous blood taken from the jugular vein of a dog, shows that dry air is capable of taking up more carbon dioxide from blood than damp air. Consequently, the lungs are less easily relieved in moist air, and the presence of carbon dioxide in the inspired air must have a proportionately more baneful influence. In 1893 Kronecker and Jordi published an account of an interesting series of experiments performed on themselves with the direct object of determining how much carbon dioxide can be breathed with impunity in inspired air. The gaseous mixture was contained in a gas-holder of 40 litres capacity. It was observed that a mixture of equal parts of air and carbon dioxide rendered breathing impossible by causing spasms of the glottis. Thirty per cent, of carbon dioxide on being breathed for a minute changed the appearance of the person experimented upon, dyspnoea resulting. A 22 per cent, mixture caused less inconvenience. With an 8 per cent, mixture the breathing was ample, being slightly more than normal. This latter observation is in complete harmony with the recent work of Hough, who shows that by breathing in a confined space of some 30 litres, healthy individuals attempt to secure an increased ventilation of their lungs by increasing the rate or depth or both of the respiratory efforts. In 1895, two years after the publication of Kronecker and Jordi's paper, J. S. Haldane gave the results of some valuable experimerits carried out upon his own person. From these he concluded that carbon dioxide is a cumulative poison, and that the symptoms depend upon the extent of saturation of haemoglobin with it. During rest it is not until the corpuscles are about one-third saturated that the symptoms become sensible; headache and respiratory distress become pronounced on reaching half saturation. Haldane further concluded that when air containing carbon dioxide is breathed some 50 per cent, of that actually inhaled is absorbed. The maximum amount of carbon dioxide that the blood can absorb from an atmosphere containing a small but fixed percentage of it, depends upon two factors, viz. the relative affinities of haemoglobin for oxygen and carbon dioxide, and the relative tension of these two gases in arterial blood. Upon breathing fresh air the disappearance of carbon dioxide from the blood is slower than its absorption was, and is due to the dissociation of carbonated haemoglobin by the mass influence of the oxygen in the pulmonary capillaries and the consequent outward diffusion of the gas through the aveolar epithelium.

From the foregoing it is clear that the effect of breathing air containing relatively large quantities of carbon dioxide is very similar to that produced by diminution of oxygen and by high altitudes, and it is no doubt due partly to the deficiency of the oxygen, but partly also to the direct influence of the carbon dioxide itself; in fact Haldane and Smith regard this latter as the more potent cause of the hyperpncea and headache which result.

The actual proportion of carbon dioxide that can be supported without inconvenience is seen to be much higher than is generally believed. It is impossible to fix with definiteness the limiting amount for safety, as this again depends upon the personal equation of the subject and upon the moisture and temperature obtaining at the time. It would appear, however, that from 8 to 10 per cent, of pure carbon dioxide may be inspired with impunity for many hours, and a slightly higher percentage for short periods. Above 10 per cent, the gas begins to have a narcotic effect, and at about 25 per cent, death may occur after several hours, although 50 per cent, may be breathed for a short time without fatal effects.
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