Chromium Exposure and Related Health Effects among Tannery Workers in Kenya

Leather manufacturing processes involve many operations, including the use of various chemicals that are detrimental to the health of workers and nearby communities.1–6 In developed countries, the production of leather has been tremendously reduced due to the closure of tanneries brought about by stringent environmental laws.1 To a greater extent, developed countries have therefore become more reliant on the supply of leather from developing countries.1,6 The latter countries, however, have inadequate environmental protection measures due to limited relevant policies and lack of enforcement of existing legislation.1,6 

Tanning processes involve several chemicals and some of them are known to be potentially carcinogenic as well as having other adverse health effects.1–15 Chromium (Cr) exposure is a major health risk in tanneries, and concentrations are supposed to be restricted.1,15 Tannery workers are mainly exposed to Cr in the inorganic or protein-bound form (leather dust).1,2,6,8 Occupational exposure to Cr is generally through inhalation, as well as dermal absorption, although ingestion is also possible where there is poor personal hygiene.1,4,6,10 

Chromium exists in two oxidation states, namely the trivalent (III) and hexavalent (VI) forms. Hexavalent Cr is more toxic than trivalent Cr and the toxicological impact is due to its oxidizing ability and high solubility.1,4,9,11–13 Chromium (VI) is capable of damaging the skin due to its high penetration power and ability to form free radicals. The species has high mobility in the environment because of its solubility in water and weak sorption onto inorganic surfaces.1,10,11 It is recommended that airborne exposure to all Cr (VI) compounds be limited to a concentration of 0.2 μg/m³ of air for an 8-hour time-weighted average (TWA).5,14 

Chromium (VI) and (III) are capable of inter-converting during chemical analysis and this behavior poses a challenge for both quantitative determination and existing regulatory policies.1,11,13,14 Cr (VI), for instance, can be reduced to more stable Cr (III) in the presence of reducing agents or oxidizable organic matter.13 The availability of Cr (III) is also a health risk due to possible conversion to Cr (VI).1,2,10 There is thus a concern over the toxic effects of Cr due to its availability and persistence during the tanning process.1–6,8 The present study was undertaken to focus on sources of high risk exposures to Cr and related health effects among tannery workers.

Ethical issues in regard to this study were addressed by obtaining relevant permits and approvals from the National Commission for Science, Technology and Innovation (NACOSTI), the Kenyatta National Hospital Ethics and Research Committee, the Ministry of Medical Services, Office of the Director Medical Services, Nairobi, and the Directorate of Occupational Health Services, Nairobi in Kenya prior to the survey. The field activities and related surveys commenced in January 2013 and ended in October 2013. Manufacturing and processing of leather, working conditions, safety behaviors, as well as related health effects of the workers and disposal of tannery wastes were investigated by the relevant research professionals and medical personnel as appropriate.

Access was permitted in one of the tanneries, which was different from the previously studied tannery.6 It had about 60 permanent and 20 temporary workers. Forty healthy office workers from a pharmaceutical company with no known exposure to Cr who matched the tannery workers for age, height, and weight served as a control group. All permanent tannery workers who were requested to participate in this study gave informed consent prior to their inclusion. Out of 60 workers, 20 were excluded because they did not either meet the criteria for the medical examination or their spot urine samples had levels in g/g creatinine per litre below 0.3 (dilute urine) or exceeded 3.0 (concentrated urine).6 

The participants were physically examined and interviewed by authorized medical personnel. The standardized questionnaire was administered face-to-face by a medical officer, with simplified medical terms that were applicable to and understood by the participants.16 It considered demographic information such as age, height, weight and personal habits such as cigarette smoking. Data on medical history, medications, hospitalizations and related respiratory and dermatological diseases were included. It was a requisite that reported corresponding diseases were observable by one or more related symptoms, such as wheezing, sinusitis, coughing and shortness of breath, or contact dermatitis, rashes and itching of the skin in the recent past (preceding three months).6 Participants who were under medical treatment for tuberculosis, cancer, and serious heart, lung or kidney complications were excluded from this study.

Ventilatory function tests were determined for each participant using a pre-calibrated portable computerized spirometer. A spirometer (Vitalograph Alpha) measuring principal flow integration (Serial No. AL 12308) and a precision syringe, (Hans Rudolph) 3 L volume delivered, were used to generate pneumotachographs that produced flow-volume and flow-time curves that were visualized on a screen.

During the lung function examination, each participant sat on a chair and tight clothing was loosened. The medical officer who conducted this exercise demonstrated the procedure since the maneuver is usually difficult and depends on the participant's cooperation and effort.6 Each participant took in a deep breath until the lungs felt full, the breath was held and lips tightly sealed around the clean mouthpiece of the spirometer. The air was blasted out forcefully and as fast as possible into the sensor until the lungs felt empty. Soft nose clips were used to prevent air from escaping through the nose. The results were then registered on pneumotachographs. A printout of the results, expressed as percentages of predictive values for each participant after adjustment for gender, race, age and height, was obtained.6,16 

The three best reproducible ventilatory function parameters were taken according to the standard of the American Thoracic Society.16 These parameters included forced vital capacity (FVC), which is the maximum volume of air that can be forcibly exhaled in one breath after a maximum inhalation.6 A decrease in FVC is therefore an indication of restrictive lung function. By contrast, forced expiratory volume of air in the first second (FEV₁) is the volume of air exhaled in the first second of the FVC manoeuvre. A reduction in the FEV₁/FVC (ratio) signifies obstructive lung function in the large airway. On the other hand, a decline in forced expiratory flow between 25–75% (FEF_25–75%) of FVC or maximal mid-expiratory flow (MMEF) indicates small airways obstruction. It should also be noted that predicted values below 80% are interpreted as an airway obstruction, restriction or a combination of the two.16 

Workers wore portable sampling pumps for a full-shift of TWA over an 8-hour period.4 Personal portable samplers made it possible to collect total Cr exposure directly for each participant. Spot urine samples were also collected and stored using standard procedures.6 A graphite furnace atomic absorption spectrophotometer (GFAAS; Analytikjena ContrAA 700, Germany) was used to analyze the total Cr in the breathing zone air and urine samples.6 

The total Cr in urine and air samples was analyzed using adequate quality control and assurance procedures. The laboratories engaged in this study had prior and regular experience in analyzing similar samples.6,7 Quality control was further ascertained by inter-laboratory comparison of the levels of Cr in 5 sets of representative breathing zone air and urinary samples that were randomly selected and analyzed using GFAAS at both the Mines and Geological Analytical and Good Manufacturing Practices (GMP) laboratories in Kenya. The levels of Cr in these sets of samples were not significantly different (P > 0.05, t-test). The range of linearity was also determined by checking the linear regression coefficient (R²) of the calibration values. It was considered acceptable when R² was 0.997 or higher. The validity of the method was further assured by method crosschecks and replication analysis. All quality control samples were analyzed in triplicate and for consistency of the results the average was taken when the relative standard deviation (RSD) was less than 7%.

Results on spot urine and breathing zone air samples for each participant were coded and statistical analysis performed using the Statistical Package for the Social Sciences (SPSS-17.0). One-way analysis of variance (ANOVA) and the Student's t-test were used for the comparison of means of Cr in diverse groups of variables as appropriate. Assumption of normal distribution for continuous variables was tested by the Kolmogorov-Smirnov test. Linear regression was used to determine the relationship between concentrations of Cr in urine (dependent variable) and breathing zone air (independent variable) samples. Multiple linear regression models were used to evaluate the effects of age, height, weight and duration of employment on Cr exposure. Qualitative data was analysed using Chi-square (χ²).

At the time of this study, there were approximately 15 tanneries that were actively operating in Kenya. They ranged from medium- to small-scale, mainly family-owned enterprises with workforces of 20 to 600. The daily production rate was about 1,000 metric tons of hides and 20,000 pieces of skins. The workers practiced conventional methods of chrome tanning, where they processed wet blues and crusts. The need for flexibility without breaking and ability to endure repeated cycles of wetting and drying were some of the reasons for using chrome. The minimum amount of chrome that was used for the optimal tanning process ranged from 4 g to 8 g per 100 g of leather and there was no recovery of Cr on site. All the operations that were observed in these facilities were manual, apart from the semi-mechanized action by drums and paddles. About 72% of the tanneries operated on one shift, while the rest were on variable shifts.

Most tanneries discharged raw effluent directly into the municipal sewers without observing relevant effluent treatment procedures. Some discharged these effluents into open fields, whereas a few (N = 5) had pre-treatment lagoons, although most of these were poorly managed. Furthermore, the majority of tanners were not informed of the legal requirements for the discharge of effluents. The washings, for example, were frequently drained through open channels and eventually became surface runoff. Solid waste was often accumulated and stored within the working environment while awaiting final disposal. Most of the tanners opted for open burning of dry solid waste in order to reduce volumes since the cost of solid waste to the designated dump site was charged as per weight of the solid waste.

The four main production units and related activities in the studied tannery are presented in Table 1. A walk-through survey in a production unit, which served as both a storage and weighing area, revealed that an average of 34 bags of chrome, each weighing 25 kg, were used daily during the tanning process. However, varied amounts of Cr were utilized in accordance with clients' specifications. The weighing procedures involved several physical processes such as tearing and opening of the bags, scooping chrome in powder form on the polythene paper and shaking the bags. The workers then stockpiled empty bags at elevated heights alongside empty chemical containers (Figure 1). The same workers swept dry floors in order to recoup scattered chrome. The general workers created additional space by collecting empty bags and containers, and burning them on open ground within the tannery (Figure 2). This was carried out on a weekly basis.

The workers in the tanning section physically mixed weighed chrome in open revolving drums and loaded them with raw materials. Liquid effluent and solid waste such as trimmings from the tanning section were conveyed through the drainage systems. In order to ease the flow of liquid effluents, general workers removed the chrome-contaminated sludge from the systems, which was subsequently dumped in an open field within the plant to dry. The dried sludge formed loose particles that had the potential of becoming airborne when disturbed (Figure 3).

Workers in the buffing section converted tanned leather into smooth leather by application of abrasives and shaving. There were emissions of leather dust in the air during this mechanical operation. The general workers eventually swept, collected and dumped the leather dust on the open ground (Figure 4). The dry solid waste that emanated from the leather tanning and processing activities was also accumulated and stored in an open field while waiting for final disposal to the municipal dumpsite. Workers further utilized the available land within the tannery, about one acre in size, to plant food crops, mainly vegetables.

Average 8-hour shifts of breathing zone air and urinary total Cr levels from tannery workers and the control group with standard deviation (SD) are summarized in Table 2. Production workers had significantly (P < 0.05) higher mean airborne Cr levels of 63.0±11.6 μg/m³ and range of 44 to 85 μg/m³ than those of the control group with a mean of 1.40±0.6 μg/m³ and range from 0.4 to 2.8 μg/m³. From the results of workers' breathing zone air in the production sections, general workers had significantly (P < 0.05) higher mean airborne Cr levels of 68.1±12.1 μg/m³ compared to those in the buffing, storage and weighing, and tanning sections with levels of 63.0±14.8 μg/m³, 62.6±7.4 μg/m³, and 53.3±6.6 μg/m³, respectively. Similarly, production workers had significantly (P < 0.05) higher mean urinary Cr levels of 31.1±7.9 μg/g creatinine with a range of 14 to 51 μg/g creatinine, compared to the control group with a mean of 0.40±0.4 μg/g creatinine and a range of 0.1 to 1.5 μg/g creatinine. Mean urinary levels of Cr of all production workers exceeded the American Conference of Governmental Industrial Hygienists biological exposure index for total Cr of 30 μg/g creatinine, and 78% of Cr levels of the general workers exceeded this limit.14,17 A significant positive correlation (R² = 0.76, P < 0.001) was also observed between urinary and breathing zone air Cr levels.

Demographic characteristics of the tannery workers and control groups are presented in Table 3. The mean age of tanners (N = 40) was 36.9±9.6 years, ranging from 27 to 57 years. Their duration of Cr-related exposure was between 2 and 10 years with an average of 5.7 ± 2.2. years. Although there were differences between the mean age, height, body weight, BMI and smoking status of the tanners and the control group, these differences were not significant (P > 0.05).

In the regression analysis model (Table 4), the association between urinary and airborne Cr and all potential confounding factors in Table 3 with respect to smoking status were established. The results did not reveal any significant (P > 0.05) association among these variables except for the duration of employment and urinary Cr levels among non-smokers (P < 0.05). However, the duration of employment was also a considerably short period that ranged from 2 to 10 years, which may not have provided a sufficient trend.

Tannery workers had significantly (P < 0.05, χ²) higher respiratory and dermatological complaints than the control group (Table 5), and 30% and 20% of tannery workers reported these complaints, respectively, compared to 10% and 7.5% of the control group, respectively. The most commonly experienced respiratory symptoms among tanners were shortness of breath (22.5%), wheezing (20%), and coughing (20%), while these symptoms were reported in only 7.5%, 5%, and 5% of the control group, respectively. In addition, 15% and 12.5% of tanners had rashes and itching of the skin compared to 5% and 5% of the control group, respectively.

Lung function values and type of impairment among the exposed and control groups are shown in Table 6. It was found that production workers (N = 40) had significantly reduced ventilatory function as follows: 17% had pulmonary obstruction, 13% had pulmonary restriction and 7.5% had both manifestations (pulmonary restriction and obstruction) compared to 5% for each listed lung function deficit in the control group, respectively.

The extensive use of Cr in industrial settings has elicited concern over the safety of workers and surrounding communities. The present study has established that all production workers had considerable levels of Cr in their breathing zone air as well as mean urinary Cr levels that exceeded the recommended limit.14,17 These results are in agreement with previous findings by Were et al.6 in a different tannery plant in Kenya which found higher mean Cr levels in both urine (35.2±12.1 μg/g creatinine) and breathing zone air (23.4±11 μg/m³) among the tanners (N = 31) compared to battery recyclers, battery manufacturers, welders and paint manufacturers. They also had a higher prevalence level of respiratory diseases, although detailed contributions of Cr from various sources within the tannery were not considered.

Our present study has shown that the average total Cr levels of the breathing zone air in the production units were about 40 times greater than those of the control group. This is not surprising since most work activities that were observed in this study had a higher potential for generating and releasing substantial quantities of Cr in the air. It was apparent that most of the workers were in direct contact with Cr at some point during the manufacturing processes. In particular, workers in the weighing and chemical storage areas were exposed to comparatively higher airborne Cr levels. This was attributed to weighing techniques that generated and released visible quantities of fine Cr particles in the air. Considerable amounts of Cr particles were also dispersed and entrained in the air while sweeping. Residual chrome particles were further observed scattering in the air whenever the empty storage bags were handled. This area did not have an exhaust system and the natural ventilation that was provided by the open door was inhibited by poor housekeeping.

The tanning processes that involved mixing of chrome and loading of materials into the open drums generated and released significant quantities of Cr-containing particles in the air. Studies have also revealed that only a third of the chrome is fully utilized during tanning processes, while the rest is discharged as tanning liquor that forms effluents. The mechanical operations such as sammying, splitting, shaving and trimming of tanned materials resulted in a combination of solid wastes and liquid effluents that contained unfixed chrome, which had the potential to contaminate the environment.

Workers in the buffing section were exposed to intense and prolonged exposure to leather dust from multiple point sources. Although there was an exhaust system in this section, it seemed ineffective since there was dust in the entire area. Fine dust was observed settling on the buffing machine and the floors. The sweeping and dumping techniques for leather dust created additional fine particles that entrained in the air and were easily blown to other areas. Domestic dogs (Figure 4) were also observed lying in this dust and were likely to contribute to the dispersion of fine dusts in the workplace environment and other areas as they roamed. It was further evident that the dogs carried home Cr-laden dust on their fur, possibly resulting in contamination in homes.

The dry tannery sludge (Figure 3) may have contributed to airborne Cr in that vicinity since the particles were loosely held and easily blown away by wind. The burning of tannery residues and empty bags also emitted visible smoke that was blown by the prevailing wind throughout the entire area. This may have polluted the air and transported Cr-bearing particles to other areas away from their point sources.1 Numerous studies are in agreement that Cr has high mobility in the environment.1 This may explain why general workers, who were involved in several activities, had higher Cr levels in their breathing zone air than the other production workers. In general, the combination of poor work practices and housekeeping alongside limited ventilation and improper disposal of tannery wastes may have contributed to elevated airborne Cr levels.

It was noted that the Nairobi River is in close proximity to this tannery, about 50 meters away from the plant. It is plausible that Cr dust could leach into the ground water and also be washed by surface runoffs during the rainy season, thereby polluting the river.18,19 Several studies have further established that the Nairobi river basin is on the receiving end of effluents. About 80% of these effluents are from manufacturing and service enterprises, making it one of the most polluted rivers in the country.18,19 Assessment of levels of Cr at 6 sites along the Nairobi River found that the levels exceeded 3 times the World Health Organization (WHO) recommended guidelines for drinking water.14 The untreated effluent from this tannery into the river may have far reaching health impacts, not only on drinking water, but also the entire food chain through irrigation.1,10,13 

Linear regression analysis established a statistically significant (P < 0.001, N = 40) positive correlation between airborne and urinary Cr levels, with a regression coefficient (R²) value of 0.76. This suggests that workers were exposed to airborne Cr mainly through inhalation.1,6,9 In contrast, studies conducted on the general population found that inorganic Cr compounds are generally absorbed through ingestion and percutaneously.1,11 However, soluble forms of hexavalent Cr compounds are readily and occupationally absorbed through inhalation and dermal contact, whereas gastrointestinal absorption only accounts for 10% of the body burden.1–6 

Workers were not adequately protected from Cr in the prevailing breathing zone air. This is not surprising given that workers were observed conducting work-related activities without the use of respirators. The findings of this study are similar to recent observations that were reported by Were et al. in another tannery that had inadequate ventilation, lack of suitable personal protective equipment and awareness that increased the risk of Cr exposure.6 In this study, exposure to Cr through direct cutaneous contact was also possible since workers were observed handling and loading tanned leather using bare hands. Their hygiene level was also low as they were seen wearing contaminated work clothes while eating. All these factors may have contributed to the Cr body burden of the workers.

Production workers had a significantly (P < 0.05) higher prevalence of respiratory and dermatological diseases that may have been caused by elevated Cr levels as evidenced in both the breathing zone air and urine compared to the control group. These findings are similar to those reported by Rastogi et al.2 Previous studies have also found a significant association between Cr exposure and dermatological and respiratory diseases among tannery workers.1–12 It is clear that chemicals that are used in leather manufacturing processes chemically alter the structure of animal hides and may have similar effects on the human skin.1,12 Even small concentrations of Cr (VI), when in contact with the skin, may trigger inflammatory skin reactions.1,12 Unfortunately, there is inadequate information with regard to occupational skin diseases among tanners despite their potential high occupational risk.1 

The respiratory tract is also a major target organ for Cr (VI) toxicity through inhalation. Shortness of breath, coughing, wheezing, perforations and ulcerations of the septum, chronic bronchitis, decreased pulmonary function, pneumonia and other adverse respiratory effects were reported as manifestations of Cr exposure.6,7,12 Hexavalent Cr has been recognized as a potent carcinogen through inhalation and there is evidence that this metal is also carcinogenic when exposed through ingestion.9,10 Epidemiological studies have further established that inhaled Cr causes increased risk of lung and sinonasal cancers.9 In addition, a population in the Oinofita municipality of Greece which was exposed to Cr (VI) in the range of 44 to 156 μg/L (N = 5) orally in drinking water exhibited a significant increase in liver cancer mortality when compared to the unexposed population.10 

Hexavalent Cr is the only form that has exhibited carcinogenic characteristics in animal studies.5 Carcinogenesis was due to the formation of mutagenic oxidative DNA lesions. A considerable increase in lung cancer mortality has also been reported among tanners who were exposed to Cr (VI) in the USA. Although the specific Cr compound that is responsible for this kind of cancer has not been identified, there is general agreement that the hexavalent form is responsible for these diseases.1 In most reported cases, however, the total Cr is usually analyzed and this often poses a challenge in differentiating Cr (VI) from Cr (III).1,6,8 

Studies have revealed that although fresh tannery sludge contains low levels of Cr (VI), the sludge, soil and leachate samples that were collected from Cr-contaminated tannery waste in Kanpur, India, had substantial amounts of Cr (VI).14 It has further been found that dry Cr (III) precipitates may be converted to Cr (VI) when heated in the presence of oxygen.13 Burning of tannery residues (Figure 2) may therefore generate Cr (VI). Trivalent Cr in the aqueous phase may also be oxidized to Cr (VI) through interaction with manganese dioxide surfaces.1,13,20,21 Speciation of Cr in the environment could change as a result of several environmental factors and Cr (III) dissolves readily in water, resulting in its bioavailability and mobility.

Our study had several limitations that included the inability to investigate the synergistic and antagonistic effects of exposures to several chemicals in tanneries, the generally low number of diseases that were investigated, as well as fairly small sample sizes. Furthermore, in the present study only total Cr was quantified and the proportion of Cr (VI) was unknown, although Cr (VI) is much more toxic than Cr (III). The health assessment using spirometry was performed on a limited number of subjects whose employment duration was short.

This study revealed various sources and occupational exposures to Cr and the related health effects among workers in a leather processing plant in Kenya. It is suggested that a combination of inadequate engineering controls, work practices and personal hygiene, together with improper management of tannery wastes led to substantial exposures to Cr and related health effects among workers. Monitoring and Cr-regulation compliance levels in tanneries alongside the relevant training support mechanisms should therefore be designed and implemented in these industries.

This work was funded in part by a grant from Blacksmith Institute for a Pure Earth. We also received financial support, approval and permits from the National Commission for Science, Technology and Innovation (NCST/5/002/R/346), Ministry of Medical Services; Office of the Director Medical Services, Nairobi and Kenyatta National Hospital Ethics and Research Committee respectively. The authors acknowledge the Directorate of Occupational Safety and Health Services (DOSHS), the Mines and Geological Department, the Kenya Industrial Research and Development Institute (KIRDI), and the University of Nairobi for providing technical support and research facilities. We are indebted to Dr. Ravi Sharma of Aga Khan University Hospital for technical support and research facilities, and the employee participants and managers of the surveyed tannery.