DIETARY EXPOSURE TO MERCURY
OF
SECONDARY SCHOOL STUDENTS
(Follow-up Report)
May 2004
Food and Environmental Hygiene Department
The Government of the Hong Kong Special Administrative
Region
This
is a publication of the Food and Public Health
Branch of the Food and Environmental Hygiene Department
of the Government of the Hong Kong Special Administrative
Region.Under
no circumstances should the research data contained
herein be reproduced, reviewed, or abstracted
in part or in whole, or in conjunction with other
publications or research work unless a written
permission is obtained from the Department.Acknowledgement is required if other parts
of this publication are used.
In 2002, the Food and Environmental
Hygiene Department conducted a study on “Dietary Exposure
to Heavy Metals of Secondary School Students” and
assessed whether there is any risk to the health of
secondary school students resulting from the dietary
exposures to heavy metals.In view of the limitations in the previous
study and the availability of methylmercury testing
locally, a follow-up study on dietary exposure to
total mercury and methylmercury was conducted to re-examine
the issue with a view to obtaining more precise estimates
of dietary exposure to mercury.
Dietary exposures to total mercury and methylmercury
were estimated by using the local food consumption
data obtained in secondary school students and the
concentrations of total mercury and methylmercury
in food samples taken from the local market specifically
for the study.More sophisticated laboratory analysis was
employed with lower limit of detection.
The results of the study showed that the dietary
exposures to total mercury and methylmercury were
all within their respective Provisional Tolerable
Weekly Intakes (PTWIs) for both average and high consumers.The dietary exposures to total mercury
and methylmercury for average secondary school students
were 0.92 and 0.35 mg/kg bw/week respectively and that
for high consumers were 2.33 and 0.87 mg/kg bw/week respectively.It can be concluded that both the average consumers
and high consumers among secondary school students
would be unlikely to experience major toxicological
effects of total mercury and methylmercury.
The results also showed that the food group
“fish” was identified as the main dietary source of
total mercury and methylmercury, and the large predatory
fish, such as swordfish and tuna, had the highest
concentration of total mercury and methylmercury.
OBJECTIVE
This is a follow-up study aiming to (i) estimate
more precisely the dietary exposures to total mercury
and methylmercury by the secondary school students
in Hong
Kong and (ii) assess the health effects of total mercury and methylmercury
resulting from these exposures.
BACKGROUND
The Previous Study
2.The Food and Environmental Hygiene Department (FEHD) conducted
a study on “Dietary Exposure to Heavy Metals of Secondary
School Students”[1]in 2002.The study aimed to determine the dietary
exposure to heavy metals of secondary school students
in Hong Kong so as to assess whether
there are any risks to their health.Three heavy metals, namely arsenic, cadmium
and mercury, were chosen for the study.
3.The dietary exposure of secondary school students to the heavy
metals were estimated in the previous study using
two sets of data.The levels of heavy metals were extracted
from the database of the Food Surveillance Programme
of FEHD and the food consumption data was derived
from the Food Consumption Survey[2]conducted
by FEHD in late 2000.
4.The previous study estimated that the dietary exposure to total
mercury for an average secondary school and high consumer
were 2.98 and 6.41 mg/kg bw/week respectively.The exposure estimate for an average secondary
school fell within the Provisional Tolerable Weekly
Intake (PTWI) of 5 mg/kg bw/week while that
for high consumer exceeded the PTWI.“Cereal and cereal products” and “vegetables”
were found to be important dietary sources of total
mercury and this finding was not consistent with those
reported in the literature.
5.It was noted that there were certain limitations in the previous
study.In
the Food Surveillance Programme, which is mainly for
enforcement purposes, analyses for mercury were determined
in form of total mercury rather than methylmercury.
The limit of detection (LOD) was 30mg/kg, which was considered
high if used for research studies.
6.A value of 1/2 LOD was assigned to samples with levels below
LOD as a conservative approach and the majority of
samples for the two food groups “cereal and cereal
products” and “vegetables” have mercury levels below
LOD.The
relatively high LOD employed by the Food Surveillance
Programme and the high amount of consumption for these
two food groups might have exaggerated the exposures
from them.
7.With the availability of methylmercury testing and the concern
over its exposure and the limitations in the previous
study, FEHD considered it necessary to re-examine
the issue with a view to obtaining a more precise
estimate of dietary exposure to mercury.More sophisticated methods of analysis
on both total mercury and methylmercury would be developed
with a lower detection limit.Food samples would be collected specifically
for the study.
10.In most foodstuffs, mercury is largely in the inorganic form.However, fish and other seafood products are
the main source of methylmercury, of which large predatory
species such as swordfish and tuna tend to accumulate
relatively higher levels.Methylmercury bio-accumulates as it moves up
the food chain, increasing in concentration at the
same time.[4]
Toxicological Effect
11.Mercury and its compounds have no known physiological functions
in animals.Their
presence in human is considered as undesirable and
may be hazardous to health.[5]
12.Organic mercury compounds are more harmful than inorganic mercury.Methylmercury is the most common form of organic
mercury and is regarded as highly toxic.
13.Methylmercury can cause adverse effect to the nervous system,
especially the developing brain. Methylmercury passes more readily through
the placenta than the other mercury compounds. Foetus exposed to methylmercury have been
found to be born mentally retarded and with symptoms
similar to those of cerebral palsy.Pregnant mothers who received low level of
methylmercury by normal adult standard may give birth
to children with serious cerebral palsy. Hence, unborn
foetus, infant and young children are more sensitive
to such toxic effects.[4]
14.Acute toxicity is often a result of occupational exposure,
and that from dietary exposure is rare.Acute effects include increased occurrence
of lymphocytic aneuploidy, discolouration of the front
surface of lens of the eyes, insomnia, tremors and
hyperexcitability.
15.There is a long latent period before early symptoms of methylmercury
poisoning including paraesthesia, malaise and blurred
vision emerge.Constriction of the visual field, deafness,
dysarthria and ataxia may develop at a later stage.The patient may partly recover from the
symptoms in a less severe case or may fall into a
coma as a result in a severe case.Damages to the central nervous system are highly
localized and affect mostly sensation, vision and
hearing. [4]
16.The Joint FAO/WHO Expert Committee on Food Additives (JECFA)
has established the PTWIs of 5 µg/ kg bw/week
for total mercury and 3.3 µg/ kg bw/week for
methylmercury respectively.Since foetus and infants may be at greater
risk of toxic effects, JECFA in July 2003 reduced
the PTWI for methylmercury to 1.6 mg/kg bw/week and this level is considered sufficient to protect the developing
foetus, the most sensitive subgroup of the population.[6][7]
17.PTWI is an estimate of the amount of a contaminant that can
be ingested over a lifetime without appreciable risk.
SCOPE
OF STUDY
18.To estimate the dietary exposures to total mercury and methylmercury,
this study covered six major food groups, namely (i)
cereals and cereal products, (ii) vegetables, (iii)
meat, poultry and their products, (iv) fish, (v) seafood
other than fish, and (vi) milk and dairy products.The other food sources, such as beverages,
were not covered in this study as they were generally
not regarded as significant sources of mercury.
METHODOLOGY
Food
Consumption Data
19.As in the previous study, the food consumption data in this
report were extracted from the Food Consumption Survey
conducted on local secondary school students in 2000
by the Department.Readers may refer to report of previous
study1
and the Food Consumption Survey report
for details.[2]
Sampling
Plan for Food Samples
20.Food samples specific for the present study were taken from
the local market according to the six food groups
as mentioned in para. 18.The food items in each food group were
selected so as to match those in the Food Consumption
Survey.
Laboratory
Analysis
21.Laboratory analysis was done by the Food Research Laboratory
(FRL) of FEHD and the analysis was conducted mainly
in form of “composite sample”.Samples of the same food types were mixed and
homogenized. For
total mercury analysis, the food samples were digested
with concentrated nitric acid and hydrogen peroxide,
and the mercury content was determined by cold vapour
atomic absorption spectrometry with amalgamation as
a pre-concentration step.For methylmercury analysis, methylmercury in
the food sample was extracted by hydrochloric acid
and derivatized with sodium tetraphenylborate, and
was analyzed by gas chromatography equipped with atomic
emission detector.The LOD for total mercury and methylmercury
for this study were 3 mg/kg and 1 mg/kg (calculated as mercury) respectively.
22.When the analytical value was below the LOD, the true value
could be anywhere between zero and the LOD.The treatment for these results was particularly
important when a large percentage of the analytical
results of a particular food group were below LOD.
23.While it may not be appropriate to assume a zero concentration
for all the samples with analytical values below LOD,
assigning the non-detects the value of LOD would,
however, grossly overestimate the dietary intake.A value of 1/2-LOD was assigned to all results
below LOD in this study.Since the levels of contaminants in food,
including mercury, usually follows a log-normal distribution,
assigning a value of 1/2 LOD to all non-detected levels
is considered as a conservative approach, especially
for food groups in which the majority of food items
have levels below the LOD.
24.For non seafood samples, only those samples with analytical
values of total mercury above LOD were further subject
to methylmercury analysis since mercury in non seafood
items is largely in the inorganic form.For these samples, a value of 1/2-LOD for
methylmercury was also assigned as a conservative
approach.
Dietary
Exposures to Total Mercury and Methylmercury
25.Dietary exposure from individual food item was obtained by
combining the consumption data and the median concentration
of individual food items.Total exposure for each student was obtained
by summing exposures from all food items.The mean of the exposures of all the students
was used to represent the average dietary exposures.The 95th percentile of the exposure
level was used to represent the exposure for high
consumers.
26.The daily dietary exposure was multiplied by seven to obtain
a weekly exposure level of an average secondary school
student.
27.The estimated weekly exposure level was then compared with
the PTWI as established by JECFA.
RESULTS
Concentration of Total Mercury
and Methylmercury
28.A total of 347 food samples were taken and combined into 115
composite samples for analysis.The median concentrations for total mercury
and methylmercury for each food group are given in
Table 1.
Table 1:Median Concentrations for Total Mercury and
Methylmercury in Six Food Groups
Food group
Total Mercury
Methylmercury
% of samples below LOD
Median concentration (mg/kg)
% of samples below LOD/ not analyzed
Median concentration (mg/kg)
Cereal and cereal products
100.0
1.5 *
100.0
0.5 *
Vegetables
91.7
1.5 *
100.0
0.5 *
Meat, poultry and their
products
94.4
1.5 *
100.0
0.5 *
Fish
0
58
0
19
Seafood other than fish
0
20
0
6
Milk and dairy products
100.0
1.5 *
100.0
0.5 *
(* a value of 1/2 LOD was
assigned)
29.Total mercury and methylmercury were detected in all the samples
from the two food groups “fish” and “seafood other
than fish”.For the remaining food groups, only a few
samples were found to contain low levels of total
mercury but none of them was found to contain methylmercury.Distribution curves of total mercury and methylmercury
concentrations in the two food groups “fish” and “seafood
other than fish” are presented in Annex I.
