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Occup Environ Med. 2007 June; 64(6): 389–395.
Published online 2007 January 16. doi:  10.1136/oem.2006.030940
PMCID: PMC2078521

Exposure to mercury in the mine of Almadén

Abstract

Objectives

To describe the process for obtaining mercury and the historical exposure of Almadén miners to mercury.

Methods

Information on every workplace and historical data on production, technological changes in the productive process and biological and environmental values of mercury was collected. A job‐exposure matrix was built with these values and the exposure to inorganic mercury was estimated quantitatively as μg/l of urine mercury. A cumulative exposure index was calculated for every worker by adding the estimates for every year in the different workplaces.

Results

In the mine, the highest exposures occurred during drilling, with values up to 2.26 mg/m3 in air, 2194 μg/l in urine and 374 μg/l in blood. Furnace operation and cleaning were the tasks with the highest values in metallurgy, peaking up to 3.37 mg/m3. The filling of bottles with mercury by free fall gave values within a range of 1.13–2.43 mg/m3 in air; these values dropped to 0.32–0.83 mg/m3 after introducing a new ventilation system. The toxicity effects of high doses of inorganic mercury on the central nervous and urinary systems have been known for decades.

Conclusions

The exposure of the workers in Almadén mines to mercury has been very high. The extremely high content cinnabar ore of the mine explains the increased concentrations of mercury in air at the work places. This, together with inadequate working conditions, explains the high mercury levels found in blood and urine during the study period.

They penetrated to the bowels of earth and dug up wealth, bad cause of all our ills, rich ores which long ago the earth had hid and deep removed to gloomy Stygian caves.

Metamorphoses, Ovidius

The mercury deposits of Almadén (Spain) have the greatest concentration of this liquid metal on Earth. Its production since historical times has been estimated in approximately 250 000 MT of metal, which represents about one‐third of the total mercury used‐up by mankind throughout history.1 The exploitation of cinnabar deposits, the main mercury ore, dates back to very remote times. The Romans were the first to extract mercury systematically, using it mostly as a pigment. The main evidence of Roman use of mercury are the great number of coins, medals, vessels and other historical objects found in the Almadenejos and Valdeazogues areas.2 On the basis of these data, we can ascertain that the Almadén mines are >2000‐years old, and the oldest known in the world.3 In 714, the Arabs invaded Iberia and the mines were passed on to the Caliphs' government. They greatly influenced the process for obtaining mercury, as shown by the marks left after their departure: Al‐mahden, which means mine, and azogue (mercury), a term used only in Spain and South America, are words of Arabic origin.

Mercury became important in the modern world due to its wide use in amalgamation processes. Around ad1550, Bartolomé de Medina developed a technique that improved or benefited silver minerals through amalgamation with mercury. This new metallurgic process, known as yard benefit, represented a significant improvement compared with the traditional melting process in furnaces, used in the silver mines of Spanish America. The lower cost, the possibility of exploiting low‐content deposits, the reduction in fuel requirements and the difficulty in obtaining mercury in the mountains where the mines were located were some of the advantages that contributed to the rapid extension of this method to the mines of New Spain (present day Mexico) and, later, to Peru. Melting was not completely abandoned, but amalgamation became the most widely used metallurgical technique. This caused a dramatic increase in the demand for mercury.1 The independence of the Spanish colonies and the introduction of mercury from California into the world market, which had been monopolised by Almadén, led to a reduction in production costs. A group of engineers promoted a large reorganisation of the mine,4,5,6 which is of interest for us, as it implied a reduction in the levels of exposure to mercury, deriving from new working guidelines. This, together with additional technological changes and hygiene improvements, allowed a significant reduction in the risk levels throughout the production process.

Currently, the mines at Almadén belong to the Spanish Government. Production of mercury has decreased notably with time (fig 11),), and the number of workers assigned to mineral extraction and production has also decreased. Mining activity ceased in 2001, with a total of 7644 tons of mineral extracted in the past decade. The metallurgical activity ended in 2004.

figure om30940.f1
Figure 1 Annual mercury production (tons) in Almadén mines.

Almadén deposits have been, and still are, extremely rich, with a mercury content of around 8%, much higher than any other mercury mine. The mine of Abbadia San Salvatore, in Monte Amiata, Italy,7 had a cinnabar content of between 0.6% and 2%. In Idrija, Slovenia,8 this value ranged from 1.5% to 3.2%. In Almadén, small drops of native mercury can be observed embedded in the cinnabar walls in most of the mines. The deposits are vertically oriented, the width growing with depth. This has led to the increasing deepening of the mine throughout history, with most of the excavations made in useful mineral due to the width of the metal veins. All this, together with mercury's physical and chemical properties, vapour emission at ambient temperatures (vapour pressure 0.17 Pa) and the deficiencies in the ventilation of the inner area of the mines, had a great effect on the mortality and morbidity of miners throughout the years, which is well described in the literature since the 16th century.9,10,11,12,13,14 Today, the occupational exposure to mercury is still important, since it is used in several industries.15 Environmental exposure is also important, mostly from food and dental amalgams; as mercury is very persistent in the environment, it may be a long‐term risk for any exposed populations.16

The aim of this paper is to describe the process of obtaining mercury and the historical exposure of the Almadén miners to mercury, and to discuss the methodological problems raised in the evaluation of the occupational exposure. Data were obtained for a cohort study on the mortality of mercury miners in Almadén, which has been included in a multicentre cohort study, coordinated by the International Agency for Research on Cancer, designed to evaluate the possible carcinogenic risk associated with the exposure to inorganic mercury.17 The description of the Spanish cohort and the methodological problems involved in its definition have been reported previously.18

Methods

The cohort was built in 1950 from the records of personnel of Minas de Almadén y Arrayanes SA. Workers joining the mine in the following years until the end of the follow‐up period in 1994 were also included.

The process of mercury extraction and metallurgy was reconstructed by a team that included an industrial hygienist from the public control office, with wide experience in the evaluation of working conditions in the mine, and the mine prevention staff. Data on mercury production, technological changes in the productive process and every work place were collected. Workers were classified according to the time since first exposure, exposure calendar date and the total length of exposure.

All the historical records on biological and environmental exposure levels from the mine were obtained from the mine medical services and the labour authority office (Centro de Seguridad e Higiene en el Trabajo de Ciudad Real). Mercury vapour in workplaces was measured by an active sampling device using hopcalite as the solid sorbent.19,20,21,22 A defined volume of air was passed through a small glass tube filled with hopcalite with battery‐operated pumps, with about 0.2 l/min flow rate. The sampling time was around two‐thirds of the work shift. The tubes were carefully sealed in plastic containers before and after they were used. The mercury in the sample was analysed using a cold vapour‐atomic absorption spectrophotometer. A volume of 20 ml blood and 100 ml urine samples were taken from workers from workplaces that were representative of the respective group, at the end of the working week. Since 1980, urine was collected in metal‐free, sterile plastic containers. The total concentration of mercury in air, blood and urine was measured by cold vapour‐atomic absorption, which included SnCl2. Urine mercury concentrations were not corrected either by specific gravity or by creatinine. The quality of the analysis was controlled using control samples in every series and with the participation of the laboratory in an external quality‐control programme of mercury analysis organised by the Centre de Toxicologie du Québec (Laval University, Québec city, Québec, Canada).23,24,25,26,27

A job‐exposure matrix was built based on the available biological indicators of mercury in blood and urine, on the reconstruction of the technological process and on the hygienic conditions in the different periods. Exposure of a given job and period (supervisor, 1985–6) was used as reference. The rest of the values were quantified in relative units to this standard; thus, a value of two implies an exposure double than the standard job‐period.

The exposure to inorganic mercury was quantified as μg/l of urine mercury. An accumulated exposure index was built for every worker by adding the estimations made for every working year at different working places.

Process used to obtain mercury

The production process included two main stages. The first one occurred completely underground and was aimed at obtaining useful mineral: cinnabar (mercury sulphide). It included all steps from the excavation of the deposits to the mineral transport to the surface, known as zafra, as well as all the maintenance and preservation tasks of the underground production area. Five tasks could be identified in this stage: excavation, extraction, fortification, drainage and ventilation. Since the end of the 19th century, all underground work was organised in two 6 h periods (2 days) per week, one working day followed by two rest days, which meant eight pay periods (working days) per month, with 48 working hours. Drilling with water was introduced in 1960, the ventilation system was improved in the 70's and, in the mid‐80's, a new extraction technology (inverted crater systems) was applied in the mid‐80's. All these measures led to a progressive reduction in the exposure to mercury in the second half of the 20th century. According to the working‐place characteristics in the underground mine, three different periods have been identified along the study interval, 1950–94: traditional system before 1972; traditional system between 1972 and 1985; new system since 1985.

Changes from the first to the second system were due to the introduction of technological improvements that modified working conditions, mostly forced ventilation, improvement of the ventilation circuit, and mechanisation of mineral loading and manipulation, which had been done manually before 1972. The exploitation system changed radically from the second traditional system to the new one, with the disappearance of most of the previous working places. Table 11 provides a detailed list of the working places, which helps to understand the nature of the changes.

Table thumbnail
Table 1 Description of the working places in the mine stage

The second stage of the process (milling or metallurgy) included mineral distillation, mercury packaging and storage. It comprised three main tasks: mineral preparation or breakdown, mineral distillation in furnace through the burning or cooking of the cinnabar, and the condensation of the resulting mercury vapour. In 1952 new Cincinnati furnaces were installed, replacing the old Spirek furnaces, which reduced the mercury exposure. In the last few years, Pacific furnaces were installed. Finally, the resulting mercury was packed and stored. The process was completed with laboratory work and in the plants of mercury‐derived products, oxides and salts, mostly. Table 22 describes the working places.

Table thumbnail
Table 2 Description of the working places in the non‐mining stages

Results

TableTable 1 and 22 provides a detailed list of the working places in the underground mine and the working places in the non‐mining stages. To complete this description, we must emphasise that workers rotations among the different workplaces were unsual, and the use of personal protection devices was sporadic until the last years of the considered period.

Throughout the studied period (1950–94), a total of 3648 miners were followed‐up and it contributed 103 728 person‐years to the cohort study. Table 33 shows the mercury values in air, blood and urine at different work places and periods. We only have biological data for 795 workers.

Table thumbnail
Table 3 Values of exposure to mercury by job and historical period

Using these levels and the reconstruction of the technological process and hygienic conditions throughout the study period, the job‐exposure matrix shown in fig 22 was built. Figure 33 gives the quantification of the exposure values, measured as μg/l of mercury in urine. An accumulated exposure index was calculated for every worker by adding the estimates for every working year in the different working places.

figure om30940.f2
Figure 2 Almadén mine's job‐exposure matrix.
figure om30940.f3
Figure 3 Estimated urinary mercury levels (μg/l) by year mine–mills of Almadén.

The workers were also exposed to crystalline silica, in both mining and milling stages. Measurements with personal devices of dust in the mills during the late 70's show levels around 5 mg/m3 (total dust) with a 10% content of free silica. There are no measurements of radon, asbestos, arsenic or diesel emissions inside the mine.

Discussion

The data included in this study constitute an effort to describe systematically describe the available evidence on exposure to mercury in mines. Previously, these had been addressed in partial and disperse studies. The quantitative exposure data show some gaps for certain work places and periods. The analysis of mercury in urine started in 1970 (uncorrected and without systematic samples); before this, the only preventive measure was medical surveillance of the exposed workers. Blood mercury analysis started in 1972, and was considered as the main exposure‐control method. Urine was analysed to obtain additional information on excretion rates. Airborne and biological exposure data were obtained by sampling different work places and miners—that is, not all the miners and work places were analysed. So, the number of workers with analytical data are a subsample of 795 workers of the cohort.

This lack of complete exposure records do not allow for the quantitative evaluation of the exposure for every job and period. However, a multidisciplinary team that included an industrial hygienist from the labour authority office, with wide experience in the evaluation of working conditions in the mine, and the mine prevention staff, made the reconstraction of data on mercury production, technological changes and exposure to mercury possible. This led to a semiquantitative assessment of the exposure levels in relative units. The results for different jobs and periods are shown in fig 22,, which is one of the few estimates of mercury exposure in mines available in the literature.

Table 33 gives the exposure of the Almadén workers to mercury, which was very high. The very high concentration of the mineral explains the high concentration of airborne mercury in all the work places, which, together with the inadequate working conditions, led to the high levels detected in urine and blood. Personal protection devices were available then, but their use was scarce and inappropriate. In the mine, the highest exposures occurred during drilling and during the work in the galleries after the blast. This explains the high values found for drillers, builders, timbermen, blasters, rubbish carriers and shovellers, with air values up to 2.26 mg/m3, 2194 μg/l in urine and 374 μg/l in blood. Paradoxically, the introduction of the new system in 1985, with relevant environmental protection measurements, did not result in a significant improvement. This can be explained by the fact that the improvements were applied in a new exploitation, but not in the old mine where the old ventilation systems and galleries coexisted.

The only relevant change to the milling process occurred in 1952, when the new Cincinnati furnaces substituted the old Spirek furnaces. Thus, the mineral milling values given in table 33 can be considered as representative for the whole period since 1952. As expected, the furnace operation and its cleaning were the tasks with the highest mercury values, with peaks up to 3.37 mg/m3, similar to those found in the mine of Abbadía San Salvatore.7 Finally, high airborne values and corresponding blood and urine values were also found in the free‐fall bottle filling, made daily and without an efficient ventilation system until 1985.

The comparison with other mercury mines is not easy, since, as previously said, there are few papers describing the exposure of mercury miners in detail, and there the exposures are for a very limited period of time.7,8,23,28,29 The most consistent comparison can be made against the quantitative estimation of exposure to inorganic mercury, measured as μg/l of urine mercury, made for the international multicentric cohort study, coordinated by the International Agency for Research on Cancer.17 Almadén shows higher mercury levels in air than Idrija (Slovenia) during mineral extraction in all the periods considered, from 1930 to 1994. This information is not available for the Abbadía San Salvatore (Italy) and Nikitovka (Ukraine) mines. However, the milling values are lower in Almaden than in the other three mines. In 1970 the production ceased in mines and mills in Idrija, restarting with minimum production in 1983. This explains the reported decrease in Idirija, to values lower than those in Almadén. In Abbadia San Salvatore, the production of metallic mercury occurred from 1893 to 1976, when mineral extraction was interrupted. Mercury production continued from secondary sources, such as waste muds and all activities ceased in 1983, with the subsequent decrease in the values for this mine.

As mentioned in the introduction, the occupational exposure to mercury is still important in many industries and the environmental exposure is not negligible. In this paper, we provide a risk assessment, and workplace exposure data for mercury during primary extraction. The data confirm that mercury miners are the occupational groups with the highest exposure in work places, reaching peak values during extraction and milling, especially for the driller, builder, timberman, rubbish carrier, shoveller, furnace worker and furnace cleaner. Retrospective data on the exposure of mercury miners provided in this paper are difficult to obtain and scarce in the scientific literature. They can be useful in several active mercury mines in USA, China, Ukraine, etc, since they are essential for correlating levels of mercury exposure and mortality for different causes.

Acknowledgements

The first part of this work was carried out at the Spanish National Institute of Occupational Safety and Hygiene, where it received institutional support, specially from Jerónimo Maqueda and Cristina Cuenca. The second part was carried out by the General Direction of Public Health in the Ministry of Health, with a grant from the National Health Research Fund (FIS 96/0942) and from the European Commission (BIOMED BMH1‐CT92‐1110 and BMH4‐CT95‐1100). The mine company (Minas de Almadén y Arrayanes SA) provided an open‐minded contribution to complete this research.

Footnotes

Competing interests: None.

References

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