The Science o f the Total Environment, 3 (1975) 267-274 © Elsevier Scientific Publishing Company, Amsterdam - Printed in Belgium
M E R C U R Y LEVELS IN W H O L E BLOOD OF S A S K A T C H E W A N RESIDENTS
C. A. R. DENNIS and F. FEHR Prairie Institute o f Enviromnental Health, 2220 Lorne Street, Regina, Saskatchewan (Canada)
(Received August 30th, 1974)
ABSTRACT The study was undertaken to establish whole blood mercury levels in the Saskatchewan population and to identify characteristics of individuals with higher blood levels. The mean blood mercury level was 6.7 ng/ml, the median level 5 ng/ml and the range 1 to 42 ng/ml. Native persons living in northern Saskatchewan who had recently eaten local fresh-water fish had the higher levels. A greater consumption of fish and higher blood mercury levels were recorded during the fresh-water fishing season.
INTRODUCTION The object of the study, carried out in 1972, was to establish baseline whole blood mercury levels in the Saskatchewan population and to identify characteristics of individuals with higher blood levels. A literature review 1-3 and general study of the uses and potential health hazards from mercury indicated that, whereas occupational, medicinal and natural background sources should all be taken into account, the probable major source of mercury intake in Saskatchewan would come from the consumption of fish, as fish is generally considered to form a significant part of dietary protein amongst residents of northern Saskatchewan, particularly those of native origin. METHODS The study was designed to identify the influence on blood mercury levels of: age, sex, occupation, medication, fish consumption, native/non-native origin and geographic location. As consumption of fish was thought to be the principal source of mercury intake, it was decided to sample during two periods of the year, prior to and immediately following ice break-up (April, May, June) and prior to and immediately after freeze-up (September, October, November). 267
The Research Committee of the Saskatchewan Chapter of the College of Family Physicians agreed to participate in the study. Family physicians, who had expressed an interest in research, were contacted requesting their co-operation in collecting blood samples from patients in each of six months during the course of routine visits to the doctor: not more than one sample to be taken from each patient during the course of the survey. Twenty-eight physicians participated in the study; their distribution was such that blood samples would be obtained from residents in most areas of the province. Doctors were divided into groups according to the approximate proportion of native and non-native patients in their practices. The number of native and nonnative patients, for which each doctor was then asked to submit samples, reflected the average distribution within the group to which he had been assigned. A questionnaire, which would give the required information relating to the parameters, was designed to be as simple as possible and to take the minimum of time to complete as this was felt to be essential to ensure completion by the physician. Materials for collection of blood samples, questionnaires and quota sheets were mailed to each participating doctor during the week previous to each of the six sampling months.
Analytical method Blood had been collected in 10-ml borosilicate tubes with sodium heparin anticoagulant. Digestion of blood samples was carried out according to the method of Uthe et al. 4. A Varian Techtron AA 120 single beam atomic absorption spectrophotometer with a Varian mercury hollow cathode lamp was used for analysis. The analytical limit of detectability was 1 ng of mercury/ml of whole blood. Duplicate analysis was carried out on every blood sample and if the deviation was more than 2 ng/ml, a further two analyses were carried out. A control taken from a unit of blood from the Red Cross Blood Bank was included with every group of samples analysed to provide a day-to-day check on variability in machine sensitivity. The mean value of repeated analyses of the control was 5 ng of mercury/ml of whole blood; 95% of the analyses fell within 5 + I ng/ml. Some samples taken in the survey were frozen and thawed before analysis. Such samples were excluded from further evaluation, as the results were not reproducible at the same level of confidence as those from the unfrozen samples. Data were examined by analysis of variance and differences in means were tested with Scheffe's Multiple F test. RESULTS
Population sample The total sample was 679, of which 28% were of native origin. Of the native sample 91% lived in an arbitrarily defined northern part of the province. These percentages are higher than one would expect to find in Saskatchewan. 268
The mean whole blood mercury level was 6.7 ng/ml. The mode (the most commonly occurring value) was 5 ng/ml. The median (the middle value of the series) was also 5 ng/ml. The frequency distribution of mercury values for the total sample is shown in Fig. 1. Approximately 10% of the sample had whole blood mercury levels above 12 ng/ml: approximately 5% above 16 ng/ml.
120
i00
8O
6o
z 40
20
i0
20
30
40
ng m e r c u r y / m l
Fig. 1. Blood mercury levels of 679 Saskatchewan residents. Mean = 6.7 ng/ml; mode = 5 ng/ml; median = 5 ng/ml.
Age, sex, occupation, medication Differences in mercury levels between males and females and among the eight 10 year age groups were not significant (Tables 1 and 2). No subjects in the study were identified as having used mercury-containing medication during the previous three months. Only 8 persons were identified as having been exposed to mercury at work during the previous 3 months; their blood levels were not higher than those who had no history of occupational exposure.
Fish consumption, natit'e/non-natiz'e origin and 9eoyraphic location Patients were questioned both on recency and on frequency offish consumption. A higher degree of specificity was found in answer to the question on recency and these data were therefore used as an indicator of local fresh-water fish consumption. 269
TABLE 1 M E A N BLOOD M E R C U R Y LEVELS F R O M A S A M P L E OF T H E S A S K A T C H E W A N P O P U L A T I O N , BY SEX Sex
Number
Mean blood mercury level (ng/ml)
Male Female Total
288 391 679
6.4 6.9 6.7
TABLE 2 M E A N BLOOD M E R C U R Y LEVELS F R O M A SAMPLE OF T H E S A S K A T C H E W A N P O P U L A T I O N , BY A G E G R O U P Age group
Number
Mean blood mercury level (ng/ml)
0-9 10-19 20-29 30-39 40--.49 50-59 60-69 70 +
52 110 115 67 70 86 90 89
6.3 7.0 6.7 7.6 6.9 6.7 5.9 6.8
Total
679
6.7
TABLE 3 A N A L Y S I S O F W H O L E BLOOD OF 679 S A S K A T C H E W A N RESIDENTS (NATIVE A N D N O N - N A T I V E ) W H O C L A I M E D LOCAL F R E S H - W A T E R F1SH C O N S U M P T I O N W I T H I N THE PREVIOUS 2 WEEKS Native
Non-native
Total in each group
192
487
With recent fish consumption number ng mercury/ml ~mean [range*
104 10.6 1-42
48 7.6 1-21
Without recent fish consumption number ng mercury/ml ~mean [range*
88 6.9 1-20
439 5.8 1-23
* Limit of detectability 1 ng/ml. 270
T h e m e a n b l o o d m e r c u r y level for the 152 persons w h o c l a i m e d to have eaten local f r e s h - w a t e r fish within the p a s t 2 weeks was 9.7 ng/ml. This level was significantly higher P < 0 . 0 1 ) t h a n the m e a n o f 6.0 n g / m l for the rest o f the sample. O f the native sample, 54% claimed to have eaten fish within the p r e v i o u s 2 weeks whereas only 10% o f the n o n - n a t i v e s a m p l e c l a i m e d recent fish c o n s u m p t i o n . T h e m e a n b l o o d m e r c u r y level for the native g r o u p was 10.6 ng/ml a n d t h a t for the n o n - n a t i v e s 7.6 ng/ml (Table 3). It was a p p a r e n t t h a t the recent c o n s u m p t i o n o f fish a n d h i g h e r b l o o d m e r c u r y levels were c o i n c i d e n t in the native group. The d a t a on f r e q u e n c y o f fish c o n s u m p t i o n s u p p o r t e d these findings. In general, b l o o d m e r c u r y levels o f residents o f the s o u t h e r n p a r t o f the p r o v i n c e were lower t h a n t h o s e o f residents o f the n o r t h e r n part. T h e p r o v i n c e was then d i v i d e d into f o u r regions for further study, a n d it was discovered that the m e a n b l o o d m e r c u r y level, 9.9 ng/ml, was higher (P < 0.01) for residents o f the n o r t h eastern rural region t h a n it was for all the rest. T h e level for p e o p l e in t h a t region w h o h a d eaten fish recently was 11.8 n g / m l ( T a b l e 4). TABLE 4 WHOLE BLOOD MERCURY LEVELS IN RESIDENTS OF 4 REGIONS OF SASKATCHEWAN, RELATED TO LOCAL FRESH-WATER FISH CONSUMPTION WITHIN THE PREVIOUS 2 WEEKS Total
With recent fish consumption
Without recent fish consumption
Total in each category
679
152
527
South, urban and rural number ngmercury/ml [mean [range*
402 5.9 1-23
29 7.4 2-21
373 5.7 1-23
number ng mercury/ml fmean [range*
77 7.2 l-I 9
16 8.1 2-19
61 7.0 l-I 8
North central rural number ng mercury/ml ~mean [range*
108 7.3 1-26
47 8.4 3-26
61 6.5 1-20
92 9.9 1-42
60 11.8 1-42
32 6.4 I-I 7
North urban
North-east rural number ng mercury/ml ~mean [range* * Limit of detectability I ng/ml. 271
Month of samplin9 The mean blood mercury levels for both natives and non-natives remained fairly constant during the months April, May, October and November; for natives they varied between 5.9 ng/ml and 6.9 ng/ml and for non-natives between 4.6 ng/ml and 5.9 ng/ml (Fig. 2). A higher consumption of fresh-water fish was recorded in June and September and blood mercury levels also appeared to rise in June (9.0 ng/ml for natives and 7.0 ng/ml for non-natives) and to be highest during September (14.6 ng/ml for natives and 8.3 ng/ml for non-natives). The mean level 14.6 ng/ml for native people during the month of September was significantly higher (P
M E R C U R Y LEVELS IN W H O L E BLOOD OF S A S K A T C H E W A N RESIDENTS
C. A. R. DENNIS and F. FEHR Prairie Institute o f Enviromnental Health, 2220 Lorne Street, Regina, Saskatchewan (Canada)
(Received August 30th, 1974)
ABSTRACT The study was undertaken to establish whole blood mercury levels in the Saskatchewan population and to identify characteristics of individuals with higher blood levels. The mean blood mercury level was 6.7 ng/ml, the median level 5 ng/ml and the range 1 to 42 ng/ml. Native persons living in northern Saskatchewan who had recently eaten local fresh-water fish had the higher levels. A greater consumption of fish and higher blood mercury levels were recorded during the fresh-water fishing season.
INTRODUCTION The object of the study, carried out in 1972, was to establish baseline whole blood mercury levels in the Saskatchewan population and to identify characteristics of individuals with higher blood levels. A literature review 1-3 and general study of the uses and potential health hazards from mercury indicated that, whereas occupational, medicinal and natural background sources should all be taken into account, the probable major source of mercury intake in Saskatchewan would come from the consumption of fish, as fish is generally considered to form a significant part of dietary protein amongst residents of northern Saskatchewan, particularly those of native origin. METHODS The study was designed to identify the influence on blood mercury levels of: age, sex, occupation, medication, fish consumption, native/non-native origin and geographic location. As consumption of fish was thought to be the principal source of mercury intake, it was decided to sample during two periods of the year, prior to and immediately following ice break-up (April, May, June) and prior to and immediately after freeze-up (September, October, November). 267
The Research Committee of the Saskatchewan Chapter of the College of Family Physicians agreed to participate in the study. Family physicians, who had expressed an interest in research, were contacted requesting their co-operation in collecting blood samples from patients in each of six months during the course of routine visits to the doctor: not more than one sample to be taken from each patient during the course of the survey. Twenty-eight physicians participated in the study; their distribution was such that blood samples would be obtained from residents in most areas of the province. Doctors were divided into groups according to the approximate proportion of native and non-native patients in their practices. The number of native and nonnative patients, for which each doctor was then asked to submit samples, reflected the average distribution within the group to which he had been assigned. A questionnaire, which would give the required information relating to the parameters, was designed to be as simple as possible and to take the minimum of time to complete as this was felt to be essential to ensure completion by the physician. Materials for collection of blood samples, questionnaires and quota sheets were mailed to each participating doctor during the week previous to each of the six sampling months.
Analytical method Blood had been collected in 10-ml borosilicate tubes with sodium heparin anticoagulant. Digestion of blood samples was carried out according to the method of Uthe et al. 4. A Varian Techtron AA 120 single beam atomic absorption spectrophotometer with a Varian mercury hollow cathode lamp was used for analysis. The analytical limit of detectability was 1 ng of mercury/ml of whole blood. Duplicate analysis was carried out on every blood sample and if the deviation was more than 2 ng/ml, a further two analyses were carried out. A control taken from a unit of blood from the Red Cross Blood Bank was included with every group of samples analysed to provide a day-to-day check on variability in machine sensitivity. The mean value of repeated analyses of the control was 5 ng of mercury/ml of whole blood; 95% of the analyses fell within 5 + I ng/ml. Some samples taken in the survey were frozen and thawed before analysis. Such samples were excluded from further evaluation, as the results were not reproducible at the same level of confidence as those from the unfrozen samples. Data were examined by analysis of variance and differences in means were tested with Scheffe's Multiple F test. RESULTS
Population sample The total sample was 679, of which 28% were of native origin. Of the native sample 91% lived in an arbitrarily defined northern part of the province. These percentages are higher than one would expect to find in Saskatchewan. 268
The mean whole blood mercury level was 6.7 ng/ml. The mode (the most commonly occurring value) was 5 ng/ml. The median (the middle value of the series) was also 5 ng/ml. The frequency distribution of mercury values for the total sample is shown in Fig. 1. Approximately 10% of the sample had whole blood mercury levels above 12 ng/ml: approximately 5% above 16 ng/ml.
120
i00
8O
6o
z 40
20
i0
20
30
40
ng m e r c u r y / m l
Fig. 1. Blood mercury levels of 679 Saskatchewan residents. Mean = 6.7 ng/ml; mode = 5 ng/ml; median = 5 ng/ml.
Age, sex, occupation, medication Differences in mercury levels between males and females and among the eight 10 year age groups were not significant (Tables 1 and 2). No subjects in the study were identified as having used mercury-containing medication during the previous three months. Only 8 persons were identified as having been exposed to mercury at work during the previous 3 months; their blood levels were not higher than those who had no history of occupational exposure.
Fish consumption, natit'e/non-natiz'e origin and 9eoyraphic location Patients were questioned both on recency and on frequency offish consumption. A higher degree of specificity was found in answer to the question on recency and these data were therefore used as an indicator of local fresh-water fish consumption. 269
TABLE 1 M E A N BLOOD M E R C U R Y LEVELS F R O M A S A M P L E OF T H E S A S K A T C H E W A N P O P U L A T I O N , BY SEX Sex
Number
Mean blood mercury level (ng/ml)
Male Female Total
288 391 679
6.4 6.9 6.7
TABLE 2 M E A N BLOOD M E R C U R Y LEVELS F R O M A SAMPLE OF T H E S A S K A T C H E W A N P O P U L A T I O N , BY A G E G R O U P Age group
Number
Mean blood mercury level (ng/ml)
0-9 10-19 20-29 30-39 40--.49 50-59 60-69 70 +
52 110 115 67 70 86 90 89
6.3 7.0 6.7 7.6 6.9 6.7 5.9 6.8
Total
679
6.7
TABLE 3 A N A L Y S I S O F W H O L E BLOOD OF 679 S A S K A T C H E W A N RESIDENTS (NATIVE A N D N O N - N A T I V E ) W H O C L A I M E D LOCAL F R E S H - W A T E R F1SH C O N S U M P T I O N W I T H I N THE PREVIOUS 2 WEEKS Native
Non-native
Total in each group
192
487
With recent fish consumption number ng mercury/ml ~mean [range*
104 10.6 1-42
48 7.6 1-21
Without recent fish consumption number ng mercury/ml ~mean [range*
88 6.9 1-20
439 5.8 1-23
* Limit of detectability 1 ng/ml. 270
T h e m e a n b l o o d m e r c u r y level for the 152 persons w h o c l a i m e d to have eaten local f r e s h - w a t e r fish within the p a s t 2 weeks was 9.7 ng/ml. This level was significantly higher P < 0 . 0 1 ) t h a n the m e a n o f 6.0 n g / m l for the rest o f the sample. O f the native sample, 54% claimed to have eaten fish within the p r e v i o u s 2 weeks whereas only 10% o f the n o n - n a t i v e s a m p l e c l a i m e d recent fish c o n s u m p t i o n . T h e m e a n b l o o d m e r c u r y level for the native g r o u p was 10.6 ng/ml a n d t h a t for the n o n - n a t i v e s 7.6 ng/ml (Table 3). It was a p p a r e n t t h a t the recent c o n s u m p t i o n o f fish a n d h i g h e r b l o o d m e r c u r y levels were c o i n c i d e n t in the native group. The d a t a on f r e q u e n c y o f fish c o n s u m p t i o n s u p p o r t e d these findings. In general, b l o o d m e r c u r y levels o f residents o f the s o u t h e r n p a r t o f the p r o v i n c e were lower t h a n t h o s e o f residents o f the n o r t h e r n part. T h e p r o v i n c e was then d i v i d e d into f o u r regions for further study, a n d it was discovered that the m e a n b l o o d m e r c u r y level, 9.9 ng/ml, was higher (P < 0.01) for residents o f the n o r t h eastern rural region t h a n it was for all the rest. T h e level for p e o p l e in t h a t region w h o h a d eaten fish recently was 11.8 n g / m l ( T a b l e 4). TABLE 4 WHOLE BLOOD MERCURY LEVELS IN RESIDENTS OF 4 REGIONS OF SASKATCHEWAN, RELATED TO LOCAL FRESH-WATER FISH CONSUMPTION WITHIN THE PREVIOUS 2 WEEKS Total
With recent fish consumption
Without recent fish consumption
Total in each category
679
152
527
South, urban and rural number ngmercury/ml [mean [range*
402 5.9 1-23
29 7.4 2-21
373 5.7 1-23
number ng mercury/ml fmean [range*
77 7.2 l-I 9
16 8.1 2-19
61 7.0 l-I 8
North central rural number ng mercury/ml ~mean [range*
108 7.3 1-26
47 8.4 3-26
61 6.5 1-20
92 9.9 1-42
60 11.8 1-42
32 6.4 I-I 7
North urban
North-east rural number ng mercury/ml ~mean [range* * Limit of detectability I ng/ml. 271
Month of samplin9 The mean blood mercury levels for both natives and non-natives remained fairly constant during the months April, May, October and November; for natives they varied between 5.9 ng/ml and 6.9 ng/ml and for non-natives between 4.6 ng/ml and 5.9 ng/ml (Fig. 2). A higher consumption of fresh-water fish was recorded in June and September and blood mercury levels also appeared to rise in June (9.0 ng/ml for natives and 7.0 ng/ml for non-natives) and to be highest during September (14.6 ng/ml for natives and 8.3 ng/ml for non-natives). The mean level 14.6 ng/ml for native people during the month of September was significantly higher (P
