Gideon Gerhardsson

Silicosis is caused by dust containing crystalline silicon dioxide: quartz, tridymite and cristobalite. Silicosisfibrosis of the lungs - is the result if more of such dust is inhaled than the body is able to eliminate. The dust must be inhaled during a sufficiently long period of time. People have always been exposed to varying amounts of stone dust. As a matter of fact, silicosis is one of the oldest occupational diseases. Consequently, the disease has been of medical interest during a long time. Extensive medical research has been devoted to it, and is still ongoing in some countries. This article is restricted to the technical aspects of the silicosis situation in Sweden.

The reported number of new silicosis cases in Sweden now are close to zero. Only infrequent single cases occur (ref 8). This is a victory for occupational hygiene engineering as a profession. In this article the strategies of the profession are outlined and exemplified with the techninical measures against silicosis in Sweden picked out as an illustrative example. Introductory, the scope and function of occupational hygiene engineering is sketched.

Occupational hygiene is by and large defined as the science and art devoted to the anticipation, recognition, evaluation, control and prevention of work-related environmental risk factors or stresses. In performing these activities the profession makes use of several scientific disciplines. However, occupational hygiene is still more of an art than a science.

Historically, occupational hygiene in Europe was a child of the growing chemical industry in the late 1800s; a medical sub-discipline, backed up by national legislation. Industrial hygiene in the United States, which emerged in the early 1900s, was in its beginning a part of the new discipline occupational medicine. Dose-response studies were needed for medical diagnoses and curative purposes. Industrial hygiene surveys at workplaces before and during thefirst world war made it clear that the health risks arising in the course of work also had to be tackled by engineering means. The association with medicine gradually became weaker and two types of non-medical hygienists appeared: industrial hygienists who concentrated on workplace measurements and industrial hygiene engineers. Once rules of thumb or permissible exposure for environmental hazards had been agreed upon, the task of undertaking and interpreting quantified workplace measurements and recognizing the need for action could be entrusted to non-medical personnel. Methods and tools could be standardised. Control, however, needed technical competence in the design and implementation of technical preventive measures. "Scientists make it known; engineers make it work", was one slogan.

The first graduate programme in the world for the training of engineers in industrial hygiene was established jointly by the Schools of Engineering and Public Health at Harvard University in 1927. Since then Harvard, John Hopkins and other Schools of Public Health in the United States have trained industrial hygienists from many countries outside America. The American training programmes have had a profound impact on the development of the profession. A number of issues have been profiled from an interdisciplinary basis. In dose-response medical research occupational hygiene measurements have gained an increasing importance.

Gradually occupational hygiene institutes were established in all industrialized countries, usually allocated to universities or other research institutes and modelled from the US experiences. American institutes patterned the Swedish National Institute of Public Health, which was sponsored by the US Rockefeller Foundation when founded in 1938, preceded by much hesitation. The American fund money was decisive for the establishment. The institute included a department for mainly medical occupational hygiene. This department was enlarged and got a technical department of its own in the National Institute of Occupational Medicine, created in 1966.

From 1938 and onward these national bodies were driving forces for medical and technical occupational hygiene. After the second world war two leading American industrial engineering hygienists, Philip Drinker and Theodore Hatch, both members of the Harvard Course Board 1927, were appreciated advisers in Sweden, particularly with regard to field studies, (ref 7). Three impacting examples were the investigation of welding risks 1948 (ref 17), the occupational related risks from petroldriven vehicles and tools 1956 (ref 14) and the silicosis project 1968 (ref 10). In these nationalwide field studies health-related emitting sources were tracked and quantified, exposure patterns outlined, and preventive measures suggested. As the insti tutes had no budget money for such types of research, the studies had to be externally financed and few supporters were available at the national level. Local company support was more easily reached and activated extended studies. On the whole, all industrial hygiene engineering projects were low budget ones.

Three early medical studies (ref 6, 15 and 18) focused the seriousness of silicosis in various occupations. The understanding penetrated only slowly. Silicosis was shaded by tuberculosis. A colleague to the medical pioneer Dr Willi Mascher, Johan Pontén, was 1930 appointed as mining physician in Boliden, by the Boliden Mining Company. His reported silicosis cases to the National Assurance Board were blamed by the Board. The Board made a request to the Mining Company urging that the doctor should be more critical in his diagnoses. Johan Pontén introduced dust-conimeter measurements and could in due time prove that reduced dust exposure, accordingly reduced the number of silicosis cases. He was a forerunner but it took time before his preventive measures influenced authorities and other companies (ref 16).

All reported cases of silicosis in Sweden between 1931 and 1956 were classified by Axel Ahlmark and coworkers (ref 1). It was pointed out that it was hard to establish the relations between exposure and the development of silicosis as long as dust measurements were not undertaken to any considerable extent. Later Ahlmark was able to finance a further survey (ref. 2). It involved dust measurements at 100 places of work which had been subject to periodical medical supervision. It also covered 70 other places of work, of which about 50 were iron foundries. Ahlmark analysed the relation between concentration and composition of dust, exposure, and influence on health (ref. 9). Furthermore, Ahlmark proposed a scale of priority for technical countermeasures based on a calculated dust index (DI). Although Ahlmark got support from the Swedish Employers Confederation and the Swedish Trade Unions Confederration he had difficulties in catching the interest at governmental level as well as from local employers and workers.

Structural unemployment of engineers later raised an unexpected opportunity to use state retraining money for silicosis abatement. Unemployed engineers could be retrained to safety engineers and such a training program would include field studies. An application was made to the authorities. And in 1968, the National Swedish Labour Market Board agreed to a new and extensive occupational hygiene survey as part of a retraining project. The Board accepted to place 4.5 million SEK at the disposal of the Institute of Occupational Medicine to investigate all places at risk, over 1,700, in the country (ref 9). Forty engineers working in team with assisting personnel created the core of the project. The work site investigations included programs for immediate action. Each company was sent a full report in two copies a few weeks after the visit, one for the employer, one for the trade union. A third copy was sent to the Inspector of Factories or the Inspector of Mines. Each report contained all results of the dust measurements and included recommendations for both immediate and long term preventive technical measures (ref 11).

Findings of major importance were later presented in special branch reports, each one covering an important industrial sector. The findings and recommendations were also put forward at public conferences, visited by a great number of people (ref 10).

In Sweden the unique Saltsjöbaden agreement between the private employers and workers organisations secured industrial peace from 1938. This cooperative philosophy was heavily attacked by the 1968s confrontation circles, triggered by the students radical movement. The efforts made in order to stop the silicosis project, however, failed. In the companies the employers and workers maintained their loyalty as to the project.

To provide an overall picture of the dust situation, the results obtained were pooled. Measurements of total dust, crystallline Si0₂ and particle size in all industries were listed in tables and figures.

The dust index (DI) contained of a few simple digits which summarized the dust situation. Beside the total concentration of dust in the air, it also included the content of crystalline Si0₂ in the dust. The calculation and interpretation of dust index were as follows for gravimetric filter sampling:
DI = A (B+5):90

where A= total dust concentration, mg/m³; B= percentage of crystalline silicon dioxide (i.e. quartz + cristobalite + tridymite) in respirable fraction.

The dust situation at a working location was in most cases considered satisfactory when the dust index was below 1.0. If total dust had more than 35% of respirable particles, the dust index should not exceed 0.5. The DI was calculated from the mean dust concentration. The DI for each activity indicated the dust situation found in that type of activity, and made a comparison with Sweden as a whole possible.

Personal dose sampling in 42 industrial sectors and branch activities covering 1,700 enterprises showed that in 28 industries, or 2/3 of the total number, the DI exceeded 1.0. In nine industries, the DI exceeded 2, and in 19 it laid between 1 and 2. Measurements with personal samplers indicated that the dust situation was unsatisfactory in practically all industries and activities, with only a few exceptions. These were the mining industry and the ceramic industry, as well as various tasks in the manufacture of abrasives and polishing materials, building elements and chemical-technical products. However, the detailed measurements showed that even these activities employed a number of people who were severely exposed to dust.

Stationary samplers were mainly used to determine the general level on the premises and as a guidance for technical changes. They were also used for determining dust concentration at a site where a certain task was being performed. Examples are the edging and trimming of stone or the cleaning of castings by grinding. Of 46 branches or activities, 15 had a dust index of 1.0 or more.

When measuring was done with personals samplers, the median value of the concentration of dust was the highest in the manufacture of roofing felt and in the production of lime and dolomite. The values were 58.5 mg/m³ and 26.0 mg/m³ respectively. In 11 of the investigated industrial sectors, the median value of the concentration of dust exceeded 10 mg/m³. Another 18 sectors, half of the examined workers, had an exposure to dust which exceeded 5 mg/m³. As a rule, personal sampling gave a higher median value for the concentration of dust than stationary sampling did. See figure 1.

Figure 1. Concentration of total dust as a function of the size of the enterprise. The diagram applies to cleaning of castings in iron foundries.

The results show that in most types of industrial activity studied, at least half of the workers examined were working under unsatisfactory dust conditions. Even in activities where the dust concentration taken as a whole was acceptable, there were some risky places. Thus, the exposure had always to be individually evaluated.

Different attempts were made to, from current conditions, calculate the cost for possible measures of reducing the dust. It was shown that measures taken in existing workshops costed 5-10 times more to carry out, compared with measures taken on the stage of planning. However, calculated costs for dust suppression in new factories were consistently high. Crushing plants may illustrate the monetary figures. About 1,500 plants for crushing rock and gravel were analysed. If dust should be reduced to a low level both in the working environment and outside it by mounting appropriate hoods on all machines and conveyers that spread dust, and by eliminating the dust diffused from roads, open spaces and stockpiles, an investment of 700-800 million SEK would be required. See table 1.

The extra cost of operating plants in a way which was kind to the environment was estimated at 200 million SEK a year. This meant that the products would become about 25% more expensive.

As an alternative, a less ambitious target was considered for small plants for crushing gravel. Most of the requirements for enclosing belt conveyors were abandoned. The investment then could be reduced to about 400 to 500 MSEK. However, the extra expense of operating all crushing plants in Sweden in a way that was not unkind to the environment still exceeded 100 MSEK a year; the products price should raise by about 15%.

Even if the dust control is carefully considered at the planning stage, the cost is considerable. The Swedish company Mataki AB erected a new plant with an output of about 700,000 tons a year of finely crushed rock. Several very advanced designs were worked out in order to prevent dust problems. The crushed products consisted almost entirely of quartzite (100% quartz) and the dust content of the air had to be kept below 0.5-1 mg/m³ at any work site. The total cost of the plant was 12 MSEK. 25% of this sum (3 mill. SEK) was spent on measures to protect the environment.

Foundries comprised one of the sectors where particularly extensive measures were necessary. A summary of the findings and actions taken many serve as a didactic example.

In 1975, the total Swedish output of castings amounted to about 550,000 tons. Of this amount, steel castings constituted about 38,000 tons, iron castings about 470,000 tons and non-ferrous castings about 41,000 tons. The total value of the annual Swedish output of castings was estimated at 2,000 MSEK. The main part of this output was sold to domestic engineering industries.

In 1975 there were 18 steel foundries and 130 iron foundries, and some 250 non-ferrous foundries. Most of the iron foundries and non-ferrous foundries were small. About 60 of the iron foundries employed less than 10 men, while about 15 had more than 100 men. In 1975 30 iron foundries with an annual output over 3,000 tons produced 80% of the total. Of the non-ferrous foundries, 150-160 had less than 10 men.

Totally some 12,500 workers were employed. From 1955 to 1975 the num- ber of workers in the foundries had fallen by about 30%, at the same time production had risen by 35%. Out of an estimated total capital cost for construction of 1,900 MSEK up to 1970, 300 MSEK were expenses for environmental protection.

During 1970-75, about 150 MSEK were invested for environmental protection, external and internal. This means about 1/3 of the total investments of 500 million SEK during the same period of time.

The time consuming introduction of olivine sand as a substitute for quartz sand meant great improvements. Olivine sand did not cause silicosis. Before the systematic prevention programs started, the silicosis risks were the greatest at steel foundries where pure quartz sand was used. At the high temperature quartz was partly transformed into tridymite and cristobalite, which are more dangerous than quartz.

In 1975 about 2/3 of Swedish steel castings were cast in olivine sand instead of quartz sand. The remaining steel foundries tried to reduce the silica dust through enclosures and ventilation. An economic complication was that olivine was three times as expensive as quartz sand. See table 2.

At the iron foundries, olivine sand was used almost entirely making ingot moulds. Of a total ingot mould production of 120,000 tons a year, some
50,000 tons were made in olivine sand. At one steel foundry, small quantities of machine castings in grey and ductile iron was cast in olivine sand. Non-ferrous castings were not cast in olivine sand in Sweden. Our contacts with other countries revealed that only a few iron and non-ferrous foundries in Norway and the USA were using olivine sand.

However, the main part of steel castings in Sweden were cast in olivine sand; 25,000 tons of a total annual output of 35,000 tons. Steel foundries used approximately 10,000 tons of olivine
sand, compared to 3,000 tons used for ingot moulds.

For high-alloy manganese steel and Cr-Ni steel, olivine as moulding material has technical advantages compared to quartz. The use of olivine for other types of cast steel depended above all on the smaller risk for silicosis.

The dust measurements emphasized the special importance of controlling dust in work with furnaces, in preparing sand and during shake out and cleaning. Core-making and moulding gave rise to less dust. The dust situation in small iron foundries was worse than in big ones. See figure 1.

The reactive measures for combating dust after a foundry was built, were much more expensive in small foundries than in big ones. In 1975 the total investment costs for ventilation in a foundry with an annual production of 1,000 tons were about 500 SEK per ton per year. The corresponding costs for a foundry producing 10,000 tons per year was about 150 SEK per ton and year.

The capital and running amounted to 0.25 SEK per kg of castings in the small foundry and to 0.08 SEK per kg in the large foundry. If castings cost 300 SEK per kg, these measures account for 8,3% of the price at a small foundry, but only 2.7% at a large foundry.

It was estimated that in 1975, the total investment for improving the environment amounted to about 20% of the estimated total capital for construction. Sand and binding agents accounted for an estimated 15% of the cost of raw material. About 250,000 tons of sand were used each year at the Swedish foundries.

The average exposure to dust for all steel foundry workers examined was found to be 10.3 mg/m³. For half of the workers, exposure exceeded 7.3 mg/m³. When comparing steel foundries using quartz sand in the moulds with those using olivine sand, exposure to dust was found to be somewhat higher at the latter.

On the average, the content of crystalline silicon dioxide (SiO₂) was 10%. At steel foundries using quartz sand or olivine sand, the figures were 13 and 5% respectively. Dust containing quartz was found at steel foundries using olivine sand. This often depended on the use of quartz sand in the facing sand or in blacking. Olivine sand itself contains no quartz, however.

The investigation showed that workers preparing olivine sand and cleaning castings were the most exposed to dust. Workers shaking out castings had the second highest exposure to dust.

The mean and medium values for exposure to dust for workers preparing sand (olivine) averaged about 30 mg/m³ and 16 mg/m³ respectively. The calculated dust index was 1.8. Extremely high exposures to dust (65-75 mg/m³) were measured during the preparation of some kinds of sand. The great difference in values found when preparing sand based on olivine or on quartz may be explained as follows. When quartz sand is used, extensive measures are taken to reduce the diffusion of dust, since the risks are known better. Again, plants that prepare quartz sand are less complicated.

During moulding shake-out and cleaning at steel foundries, there were no great differences in exposure to dust between workers using quartz sand and those using olivine sand. The exposure to dust for workers doing cleaning and shakeout was too high in most cases. This applied especially to workers at steel foundries using quartz sand. Here, the higher content of crystalline silicon dioxide in the dust - 17% during shake-out and 13% during cleaning - caused dust indexes of 1.7 and 2.0 respectively.

The average exposure for all examined workers was 19.5 mg/m³. It exceeded 12.7 mg/m³ for half of the workers. Of the total number of workers examined 84% had an exposure to dust exceeding 5 mg/m³. The average content of crystalline silicon dioxide was 13%.

The heaviest exposure to dust occurred during work with furnaces and ladles. The other tasks could be arranged as follows, with relatively small differences between them: cleaning the premises, cleaning castings, shake-out, preparing sand and general foundry work. It was evident from the investigations that cleaning with portable machines created considerably higher dust levels than stationary grinding machines did. Probable reasons were that portable machines held in the hands were mainly used for grinding coarse castings, where equipment for pre-cleaning was often lacking, and that sand burns hard more often on coarse castings. It was noted that exhaust hoods were more common on stationary grinding machines.

The average exposure to dust for all workers examined was 12.1 mg/m³. For half of these workers, average exposure was more than 9 mg/m^3. The heaviest exposure took place during blasting (55.2 and 29.0 mg/m³ respectively). The median exposure to dust during cleaning averaged 7.6 mg/m³. The corresponding figure for iron foundries was 13.9mg/m³. The average SiO₂ content was 13%.

Blasting with abrasives was used for many different purposes. Air blasting machines used compressed air and centrifugal forces to accelerate the abrasive which could be metallic or non metallic.In blasting of surfaces, dust was produced from abrasive, workpiece and surroundings. To reduce the inconvenience of dusty air to the operator and other people nearby, the concentration of dust had be kept low by means of suitable choice of abrasive and apppropriate ventilation.

Quartz sand was the most widely used abrasive as sands of satisfactory quality is relatively cheap. See table 3.

A great part of the blasting products consisted of objects of such a size and shape that they had to be cleaned by manual blasting. The size, the design and the location of such objects generally made reuse of abrasive impractical. Olivine as a substitute for quartz was recommended for different kinds of dry blasting e.g. bridges and ships. Olivine also had excellent properties in water-sand blasting and in steam sand blasting.

Measurements made in order to compare the cutting speed of different abrasives revealed that the area cleaned per hour (hot-rolled steel) was roughly the same for quartz, olivine and slags (10-13m²/h). The quantity of abrasives used per hour was very similar for different slags but the amount was doubled if quartz was used. If olivine was used, the quantity had to be multiplied by a factor of 3 (70, 150 resp 250 m²/h). Consumption of abrasive per m² was higher for olivine and quartz than for the slags (20, 13 resp. 7 kg/m²). The tendency to create dust was lower for the slags compared to quartz and olivine. Copper slags were the least dusty. The black colour of the copper slags made their dust more visible to the human eye, compared to the nearly invisible transparent dust from quartz.

In blasting operations the operators’ exposure should be kept below the threshold limit value for inert dust which in Sweden at that time was 10 mg/m³, calculated as total dust. Olivine was not as dusty as quartz, see table 4, but the absence of silicosis risks could make the handling less careful.

In total these field studies pinpointed that extensive systematic technical measures had to be taken in order to eliminate the risks of silicosis. The greatest possible efforts should be made to improve the conditions at existing places of work and expert advice was needed before reengineering old enterprises or building new ones. Above all, it was important to motivate measures based on monetary cost alternatives. Important actors accepted engagement. The Swedish Employers´ Confederation and The Swedish Trade Union Confederation gave over the coming years continuous advice to enterprises and industrial sectors. Several branches showed a tripartite interest in the silicosis problem via their professional organizations, (ref 11). The technical sections of the occupational health services in the companies gave high priority to the abatement of silicosis.

The guidelines issued by the Silicosis Project were widely spread. Guidelines were written for emission and imission control, periodic dust measurements, technical check ups.

The resources of the National Swedish Board of Occupational Safety and Health were extended which particulary gained the small and medium-sized enterprises (ref. 5). The government placed 11.5 MSEK at the disposal of the Industrial Safety Inspectorate for a five-year follow-up of the Silicosis Project. Technical experts on dust elimination were employed. The Inspectorate was able to make a technical follow-up of 3,000 places of work.

The projects’guidelines for emission control were based on the strength of the sources. Measuring the specific dust production - the emission rate per production unit or time unit - had several advantages. It opened the opportunity of controlling a machine tool or operation with regard to its emission. It enabled requirements to be set for the maximum permissible emission from different types of machines. When appropriate, point sources were quantified in a test chamber - a tent of nylon fabric - in which a laminar air flow was maintained. The air flow gathered airborne particles less than 10 m emitted by the technical operations in the tent.

A typical specific requirement could be worded thus: "A portable tool for grinding concrete must be equipped with an exhaust blower or other facility for eliminating dust of such quality that the amount of dust emitted in the working place (with a particle size of less than
10 m ) not exceeds 25 mg/minute during normal grinding". The permissible emission values set should ensure that the exposure for a worker who grinded concrete not exceeded the immission threshold limit value. An exhaust system (Low Volume-High Velocity) was invented by one of the teamleaders, Göran Isaksson, in the course of the project (ref. 12). It proved to have a very good dust reducing effect e.g. for cleaning ingot moulds.

Relatively simple methods could be used to determine the specific production of dust from several point sources, e.g. at sites for drilling rock or grinding sites for cleaning castings. The production of dust from surface sources such as belt conveyers or stockpiles of crushed material was more difficult to quantify, so also mobile sources such as wheeled traffic. A program was made for remedial or preventive measures.

1) Ahlmark A, Bruce T, and Nyström Å: Silicosis and other pneumocnioses in Sweden.
Scand Univ Books, Stockholm (1960).

2) Ahlmark A: Silicosis in Sweden. Studia Laboris et Salutis. National Institute of Occupational Medicine (Stockholm) 1967.
Swedish

3) Ahlmark A: The Swedish National Pneumoconiosis Register. Establishment and use.
Läkartidningen 62: 198-202 (1965). Swedish.

4) Ahlmark A, and Gerhardsson L: The Silicosis in Sweden after 1930.
Arbete och Hälsa 1981:1. National Swedish Board of Occupational Safety and Health. Stockholm (1981).
Swedish.

5) National Board of Occupational Safety and Health: Directions for Iron
Foundries. Stockholm, No 76 (1970). Swedish.

6) Bruce T: Die Silikose als Berufskrankheit in Schweden. Eine klinische und gewerbemedizinische
Studie Acta Med Scand 110, suppl 129 (1942). German.

7) Drinker P and Hatch T: Industrial Dust. McGraw-Hill. New York (1936).

8) Data from the Swedish Work Environment Authority, 2000. In Engstrand K: Swedish initiatives in
international development in Occupational Health. Int J Occup Environ Health 7:129 (2001).

9) Gerhardsson G, Andersson A, Engman L, Isaksson G, Magnusson E, and Sundquist S:
The Final report of the Silicosis Project. Part1. Methods and measuring strategy (1974) Investigation Report AMT 103/74-1. National Swedish Board of Occupational Safety and Health, Swedish.

10) Gerhardsson G, Engman L, Andersson A, Isaksson G, Magnusson E. and Sundquist S:
The Final report of the Silicosis Project. Part 2. Aim, scope and results (1974.) Investigation Report
AMT 103/74. National Swedish Board of Occupational Safety and Health, Swedish.

11) Gerhardsson G, Isaksson G, Andersson A, Engman, L, Magnusson E and Sundquist S:
The Final report of the Silicosis Project. Part 3. Preventive measures (1974). Investigation Report
AMT 103/74-3. National Swedish Board of Occupational Safety and Health.

12)Gerhardsson G, Isaksson G, Andersson A, Engman L, Magnusson E, and Sundquist S: Dust con trol in the working environment. (Silicosis) ILO Occup Safety Health Series no 36. Geneva (1977).

13) Gerhardsson L, and Ahlmark A: A 50 years survey and follow-up of silicosis in an industrial country:
Mineral exposure and pneumoconiosis Am Occup Hyg 32:697-704. Suppl 1 (1988).

14) Gerhardsson G: Employers examining the working environment. Swedish Employers Confederation, Stockholm (1992). Swedish.

15) Mascher W: About Silicosis. Sw. National Association against tuberculosis, kvskr 24 (1929). Swedish.

16) Pontén J: Health and Medical Care at the Boliden Mining Company 1930-1960. Boliden Mining Company. Stockholm (1975) Swedish.

17) Thrysin E and Gerhardsson G: Regarding the composition of fumes and gases at arch welding. National Institute of Public Health. Stockholm 1951 Swedish.

18) Wulff H.B.: Studies on the pathogenesis and the pathological anatomy of silicosis, based on an examination of the lungs of stonecutters, particularly from Hardeberga Stone Quarry in the province of Scania, Sweden. Acta Pathol Microbiol Scand 11: 389-441 (1934).