Prof. R. A. Babayants (Leningrad)

Sulfur Dioxide in Urban Air

From the Air Hygiene Department of the Leningrad Research Laboratory of Municipal Hygiene (Director: Dr. S. A. Kechek)

Of all the industrial gases found in the atmosphere of populated areas, sulfur dioxide is of particular concern to public health authorities because it is released in substantially greater quantities than other gases in industrial cities. It enters the air wherever mineral fuels and peat, which contain from 0.2 to 5–6% sulfur, are burned. When fuel with an average sulfur content of about 3% is burned, approximately 5% of its weight is released into the atmosphere as sulfur dioxide. In addition, \(\mathrm{SO_2}\) enters the air as a by-product in the manufacture of sulfuric acid, cellulose, glass, and ultramarine, as well as in the roasting of sulfur-bearing ores, pickling, and other processes. Sulfur dioxide is used to bleach woolen cloth, silk, paper pulp, feathers, straw, wax, bristles, and horsehair, and also to disinfect fruit, preserved foods, and other products. If these production processes are not sufficiently sealed, they contaminate the atmosphere with considerable quantities of \(\mathrm{SO_2}\). Much \(\mathrm{SO_2}\) enters the air with various types of smoke. Thus, according to Wislicenius, individual types of smoke contain the following quantities of \(\mathrm{SO_2}\), in \(\mathrm{mg/m^3}\): from burning wood—0; from cellulose mills—655; from locomotives—756; from factories and plants in

on average, 2,934; sulfuric acid plants, from 3,406 to 11,790; ultramarine plants, from 13,100 to 91,700; copper ore smelting, 44,540; and pyrite roasting, 222,700. Over the past 10 years, the quantities of SO₂ emitted into the air have increased. For example, production of sulfuric acid and superphosphate in Russia in 1913 amounted to only 186,000 tons. During the first Five-Year Plan, five chemical plants were reconstructed and expanded, and five large chemical combines were newly built. This increased annual production of sulfuric acid and superphosphate during the first Five-Year Plan from 186,000 to 1,112,000 tons (Weisbain, Ioffe, Tyshkovsky). One of the latter plants, the Nevsky Chemical Combine, emits up to 12,000–13,000 kg of SO₂ into the atmosphere per day (Irbe-Kagan).

Hygienists and public health workers are primarily concerned with: 1) the quantities of SO₂ in the air of populated areas; 2) the minimum gas concentrations that already have a harmful effect on human health and plants; and 3) the size of protective zones around the principal sources of air pollution by sulfur dioxide.

Sulfur dioxide is an irritant gas. In atmospheric air, in the presence of moisture, it is converted into sulfuric acid, which causes burns to tissues (foliage) and corrosion of metals. The data collected on this subject (Lazarev and Astrakhantsev, Flury and Zernik, Babayants, and others) permit the following conclusions. Low concentrations of SO₂ cause irritation of the mucous membranes in humans. As concentrations increase, hoarseness, bronchitis, pain and a feeling of tightness in the chest, conjunctivitis, difficulty speaking and swallowing, and sometimes vomiting are observed (Hamilton). Higher concentrations lead to acute bronchitis, cyanosis, disturbance of consciousness, and finally death as a result of reflex spasms of the glottis (Carrozzi).

Hamilton considers pulmonary edema a possible consequence of SO₂ poisoning, while Flury and Zernik believe that asphyxia may result from circulatory arrest under the influence of SO₂. Poisoning by the latter leads to purulent bronchitis and liquefaction of the lymph glands, as established at autopsy (Crecelius).

Until very recently, hygienists studied SO₂ concentrations and their harmful effects solely from the standpoint of protecting workers employed in industries where considerable quantities of sulfur dioxide are released. It is therefore understandable that the hygiene literature reports the harmful effects of high SO₂ concentrations that are not encountered in the open atmosphere.

Table 1 presents some of the published material on this question. We see that the results of experimental studies by foreign authors do not agree. Whereas Fieldner and Katz, as well as Henderson and Haggard, take a concentration of 8–13 mg/m³ as the odor detection threshold, Smolczyk and Cobler report a “faint odor” at 4 mg/m³. Lehman considers a concentration of 20–30 mg/m³ not very distressing, whereas Zeldner, Katz, Henderson, and Haggard find that it causes eye irritation and, after several hours, mild symptoms of poisoning. Almost all authors note that the observed bodily reactions relate to short exposures and that the harmful effects of these concentrations intensify as the duration of exposure increases.

From the standpoint of protecting atmospheric air quality, the results of experimental studies of the harmful effects of low SO₂ concentrations during prolonged exposures—lasting several months—would be of greatest interest. However, we found no mention of such experiments in the literature. In this respect, the results of experimental studies by a number of authors on the effects of low SO₂ concentrations on plants deserve serious attention. Depending on the concentration, sulfur dioxide at first inhibits the assimilation of SO₂ by plants—

Table 1

Harmful effects of SO₂ on humans and animals at different concentrations

Table 2

Harmful effects of SO₂ on plants

…then impairs their respiration, has a destructive effect on protoplasm, bleaches leaves and the delicate colors of flowers, causes partial or complete scorching of needles and leaves, and finally kills the plants. The available data on this subject are presented in Table 2, which shows that an atmospheric SO₂ concentration of 2.6 mg/m³ adversely affects vegetation and may cause the death of trees.

Having established from the literature the minimum SO₂ concentrations that harm living organisms or, as Henderson puts it, alter the “environment of life,” we turn to the data on SO₂ quantities found by Soviet authors in the atmospheric air of various locations.

A considerable number of sulfur dioxide measurements concern the air in industrial workplaces. In recent years, however, a number of studies have investigated SO₂ concentrations in the outdoor air of cities.

Table 3

Concentrations in the air of industrial workplaces

The material presented in Table 3 is significant not only from the standpoint of occupational health but also for the hygiene of populated areas. Air from the industrial workplaces listed in the table, containing enormous quantities of sulfur dioxide, is discharged into the atmosphere of cities. These concentrations also make it possible to assess critically the data obtained for the outdoor atmosphere.

As regards the concentration of sulfur dioxide in the atmospheric air of cities, the material on this subject collected by Uglov, Yakovenko, and Babayants, covering both Soviet cities and certain foreign ones, can be divided into two groups. The first consists of figures for the \(\mathrm{SO_2}\) content of the air in individual cities; the second, of sulfur dioxide concentrations by zone around certain factories.

The concentrations of \(\mathrm{SO_2}\) in the air of individual foreign cities are shown by the following data (in mg per \(1\ \mathrm{cm^3}\)): Berlin, 1.48 to 1.5 and, on one occasion, 32; Liège, 0.6 to 1.5, and in the city center, 0.5 to 5; London, 0.14 in summer, 0.5 in winter, and 2.8 to 9.2 on foggy days; Manchester, 1.9 to 9.4 in clear weather and 7.4 to 14.1 in foggy weather; Chicago, 0.64 in the industrial district, 0.96 in the railway district, 0.4 in the residential district, and 0.06 to 0.2 in parks and outlying areas; a small English town, 0.3.

In the USSR, the study of \(\mathrm{SO_2}\) concentrations began earliest in Ukraine, using a simple instrument proposed by Ost and improved by Bamberger, Nussbaum, and Liesgang. The instrument is based on the absorption of \(\mathrm{SO_2}\) by filter paper impregnated with a mixture of potash, glycerin, and water in a ratio of \(1:1:2\). The quantity of \(\mathrm{SO_2}\), determined gravimetrically, is expressed relative to the absorbing area for a known number of hours of exposure.

The principal drawback of this method is that it determines the quantity of \(\mathrm{SO_2}\) in a known volume of air. Moreover, as tests in Moscow (Dokuchaeva), Odessa (Zheleznyakova), Baku (Shifrin), and to some extent Leningrad showed, the method can yield identical quantities of \(\mathrm{SO_2}\) at entirely different airborne concentrations, because gas absorption depends on the movement of the air mass. In addition, the rate of absorption decreases as the paper dries; and, most importantly, the extremely dirty paper and the large amounts of dust and wash water unavoidable in this determination cast doubt on the accuracy of the results obtained.

A hygienic assessment of the \(\mathrm{SO_2}\) content of air can, of course, be made only on the basis of its concentration. For these reasons, we present below only the results of determinations of airborne \(\mathrm{SO_2}\) by the aspiration method, which is used by most atmospheric researchers and was recommended by the conference of air-hygiene specialists organized by the All-Union Institute of Municipal Hygiene and Sanitation in Moscow on February 10–14, 1940. Such studies include those of \(\mathrm{SO_2}\) concentrations in Moscow, Leningrad, Molotov, Dnepropetrovsk, and Stalino.

In Dnepropetrovsk, Donich made 35 determinations of \(\mathrm{SO_2}\) in December 1928 and January 1929 at distances of 0.43–8.3 km from a metallurgical plant. The following concentrations were obtained (\(\mathrm{mg/m^3}\)): in 6 determinations, 0; in 8, 50 to 80; in 4, 0.5 to 5.0; in 4, 115 to 131; and in 13, 5.0 to 25.0; the average was \(36\ \mathrm{mg/m^3}\).

As individual data show, at considerable distances from the plant and with the same wind direction, negligible amounts of \(\mathrm{SO_2}\) were found in some cases and enormous quantities in others. Moreover, the enormous concentrations (\(25\) and \(50\ \mathrm{mg/m^3}\) at a distance of 3–5 km from the plant, 60–131 mg at a shorter distance, and an average of \(36\ \mathrm{mg/m^3}\)) give rise to serious doubt and are apparently explained by some methodological peculiarities of the work. In Stalino, 19 determinations of the concentration of \(\mathrm{SO_2}\) in the air were made in November and December at distances of 1 to 5 km from the source of pollution. The results of these determinations, with averages calculated by zone, are given in Table 4. It does not include the determinations (\(19.1\ \mathrm{mg/m^3}\) at a distance of 5 km, 7.44 mg

Table 4

SO₂ concentration in the air of Stalino

at 4 km; 11.6 mg at 2.5 km and 12.1 mg at 3 km), which we considered incidental.

Among the Ukrainian studies, mention should also be made of the SO₂ concentration in the air of Kharkov, 0.1–1.3 mg/m³ (Yakovenko). In Molotov, a methodological check determined the concentration of SO₂ in the air to be 0.23 mg/m³ in summer, 0.47 mg/m³ in winter, and 0.39 mg/m³ on average (Ryazanov).

In Moscow, the Erisman Institute determined SO₂ concentrations around the Voskresensk Chemical Works, and the All-Union Institute of Municipal Hygiene did so around the Kashira Power Station over a period of two months in summer. The mean results of these studies are presented in Table 5.

Table 5

SO₂ concentration by distance zone from the Voskresensk Chemical Works and the Kashira Power Station

Finally, in 1938 and 1939, the air hygiene department of the Leningrad Scientific Research Institute of Municipal Hygiene conducted extensive studies of the zonal dispersion of industrial gases around chemical plants (Krasny Khimik and the Nevsky Chemical Works), the Vorovsky Cement Plant, and the large Kirov Machine-Building Plant (Irbe-Kagan, Kolomeets, and Babayants). The studies were conducted in spring, summer, and autumn around these plants along selected compass bearings, on the windward and leeward sides, at distances of 250, 500, 1,000, and 2,000 m from the plants. At all leeward points and at one windward control point, 100- and 300-liter air samples were collected simultaneously by the aspiration method every six-day period, or every other such period. In all, 1,209 air samples were collected around the two chemical plants and the Kirov Plant for the determination of sulfur dioxide, nitrogen oxides, and fluorine. SO₂ was determined in 493 samples: 265 were taken around the Nevsky Chemical Works, 148 around Krasny Khimik, and 80 around the Kirov Plant. At the same time, the quantity and composition of particulate air pollution around Krasny Khimik, the Vorovsky Cement Plant, and the Kirov Plant were studied continuously for one year at the same distances, at 37 sites, using the sedimentation method with 15-day exposure periods.

All these studies were undertaken at the suggestion of the architectural and planning department of the Leningrad Soviet to determine the possibility of reducing—

¹ By the permeable-screen method.

…of the established 2-kilometer protective zone and the use of the land around the plant for construction. The research results were subjected to careful statistical analysis, both by zone and by individual or combined directions according to the wind rose. Average data on sulfur dioxide concentrations around these three plants are presented in Tables 6, 7, and 8.

Table 6

SO₂ concentrations around the Kirov Plant, in mg/m³

Table 7

SO₂ concentrations around the Nevsky Chemical Combine, in mg/m³

Table 8

SO₂ concentrations at the Krasny Khimik Plant, in mg/m³

In assessing the sanitary and hygienic significance of all the citywide and zonal SO₂ concentrations presented above, the first question that arises is what minimum concentration of SO₂ may be considered permissible in urban atmospheric air.

As is well known, there are no standards on this question, nor even any reasonably specific proposals. The SO₂ concentration of 20 mg/m³ (0.02 mg/L) permitted under OST (90014–35) for industrial premises, based on short-term exposure of adult workers, is of course entirely unsuitable for the outdoor atmosphere of cities. As we saw above, according to Lehman (see Table 1), this concentration is “distressing,” albeit not severely so, while according to Henderson and Haggard it causes throat irritation.

In populated areas, only those concentrations of SO₂ in the air that are completely harmless to people can be considered permissible,

animals, vegetation, buildings, and so on. They must be many times lower than the doses that exert any harmful effect. Table 2 shows that, according to Stöckhard, an atmospheric concentration of 2.6 mg of SO₂ per m³ already has a harmful effect on plants and, according to Henderson and Haggard, can kill trees. The more highly organized human tissues and organs would apparently be more susceptible to the harmful effects of SO₂ than animals and plants. On these grounds and on the basis of the data presented in Tables 1 and 2, we can say a priori that a concentration of 2.6 mg/m³ of SO₂ is not the minimum that has a harmful effect on human health, especially that of children.

These considerations lead us to accept the view of the Erisman Institute that only hundredths of the concentrations of SO₂ harmful to living tissues are permissible in the atmospheric air of populated areas. Relaxing this requirement tenfold relative to the minimum dose reported by Stöckhard, Henderson, and Haggard to be harmful to living organisms, we obtain 0.26 mg/m³. As we saw above, SO₂ concentrations close to this value have been found in relatively clean places: in Kharkov (0.1), in Molotov in summer (0.23), in London in summer (0.14), in parks and the surrounding areas of Chicago (0.06–0.2), at a distance of 3 km from the Voskresensk Chemical Combine (0.2) on the windward side, 1,000 m west of the Krasny Khimik plant in Leningrad (0.39), and in a small English town (0.3).

Thus, the data presented above on sulfur dioxide concentrations in the air of various places, the analysis of experiments and observations on the effects of the gas on humans, animals, and plants, and the foregoing considerations suggest that an atmospheric SO₂ concentration of 0.26 mg/m³ or, rounded, 0.30 mg/m³ should apparently be regarded as the limit for populated areas. This hypothesis must be carefully tested by means of extensive measurements in clean and more or less polluted locations in industrial cities, as well as by biological experiments to determine the minimum doses (less than 2.6 mg/m³) that cause pathological effects in animals and plants upon prolonged exposure.

It should be noted that the average results presented above, based on a vast number of measurements of atmospheric SO₂ concentrations around the three chemical plants mentioned, fully substantiate the two-kilometer protective zone adopted by the OST.

Studies around chemical plants in Leningrad found the following average sulfur dioxide concentrations in individual zones, based on 493 samples: at a distance of 1,000 m, from 0.89 to 0.92 mg/m³, and at 2,000 m, 0.87 mg/m³. If maximum concentrations are considered, those at 1,000 m ranged from 1.75 to 2.03 mg/m³ and even reached 6.6 mg/m³, while at 2,000 m they were 1.23 mg/m³ (Tables 7 and 8). Around the Voskresensk Chemical Combine near Moscow, the following average concentrations were found: 0.5 mg/m³ at 1,000 m, 0.3 mg/m³ at 2,000 m, and 0.2 mg/m³ at 3,000 m.

The harmful effects of these SO₂ concentrations on living organisms may be judged from the results of a dendrological survey conducted by our department in 1938 and 1939 of areas of vegetation 150–2,600 m from the Krasny Khimik and Nevkhimkombinat plants. Around the latter, at distances of up to 2,000–2,600 m, 75–100% of the leaf surface of lindens, larches, ashes, and even birches and oaks had been scorched; apple trees, willows, jasmines, poplars, and other species showed 30–75% damage at distances of up to 2,300 m from the plant (Loginova). At 1,000–1,600 m from Krasny Khimik, damage was found to 26–65% of the leaf surface of currants, hazels, rowans, lindens, apple trees, honeysuckles, ashes, birches, and others (Loginova). Specially con-

…a survey conducted by our department of the area around the “Krasny Khimik” plant, by individual zones (150, 250, 500, and 1,000 m from the plant), showed that residents complained of gases, of being unable to remain indoors on certain overcast days, of the rapid deterioration of nickel-plated furniture and iron objects, of poor survival among small animals (goats), of throat irritation, and so forth (Bochkareva).

All these data support the conclusion that the 2,000 m radius established by the OST around chemical plants as a protective zone in which the construction of residential and public buildings is prohibited is a minimum and cannot be reduced under any circumstances. To protect premises outside this zone, special barriers for the absorption of gases must be established within it, in the form of plantings of gas-resistant species, bodies of water, fountains, and the like. Within this zone, warehouses and other buildings with minimal staffing may be permitted for the storage of materials unaffected by gases.

In cities where the specified protective zone cannot be maintained around chemical plants—as is the case in Leningrad for both plants named above—they should be relocated beyond the city limits, as envisaged in the long-term plan for the reconstruction and sanitary improvement of Leningrad.

The State Sanitary Inspectorate must impose strict requirements for sealing production processes that release \(\mathrm{SO_2}\) and for cleaning flue gases containing it.

The results of the study of \(\mathrm{SO_2}\) concentrations around the Kirov Plant and the Kashira Power Station (see Tables 5 and 6), whose production activities are atypical, permit conclusions relevant only to local planning.