Risk
Assessment Studies
Report No. 18
Chemical Hazard Evaluation
DIETARY EXPOSURE TO LEAD
OF
SECONDARY SCHOOL STUDENTS
May 2005
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 (FEHD) 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 FEHD.Acknowledgement
is required if other parts of this publication are used.
This
study estimated the dietary exposure to lead of the secondary
school students in Hong Kong, assessed the associated
health risk and explored measures to reduce the lead exposure.
Dietary
exposure to lead was estimated by using the local food
consumption data obtained in secondary school students
in 2000 and the concentrations of lead in food samples
taken from the local market.Laboratory
analysis for lead was conducted by the Food Research Laboratory
of the Food and Environmental Hygiene Department.
The
dietary exposures to lead for average and high consumers
of secondary school students were 1.98 and 5.09 mg/kg bw/week respectively.Both
levels were well below the Provisional Tolerable Weekly
Intakes (PTWI) of 25 mg/kg bw/week
for lead.It can be concluded that both the average and high consumers
of secondary school students are unlikely to experience
major toxicological effects of lead.
The
results also showed that the food group “vegetables”,
particularly leafy vegetables, was identified as the main
dietary source of lead.Surface contaminated lead could be effectively removed
by thoroughly soaking and washing of leafy vegetables in
water.Lime preserved egg and oysters were found to contain
high concentration of lead and they may be significant
dietary sources of lead if they are consumed in large amount.
Because
of the ubiquitous nature of lead, low levels of lead in
foods may be unavoidable.Food
trade is recommended to observe good agricultural and manufacturing
practices to minimise lead contamination of foods.Members
of the public are advised to wash vegetables thoroughly
before cooking and have a balanced diet to avoid excessive
exposure to lead from a small range of food items.
OBJECTIVE
This
study aims to determine the dietary exposure to lead by the secondary
school students in Hong Kong, to assess the associated health risk and to explore measures to reduce dietary exposure
to lead.
BACKGROUND
2.The Food and Environmental Hygiene Department (FEHD) has conducted a study on “Dietary
Exposure to Heavy Metals of Secondary School Students” in
2002, in which three heavy metals, namely mercury, arsenic and
cadmium were studied, and a follow-up study on “Dietary Exposure
to Mercury of Secondary School Students” in 2004.In
view of the adverse effect of lead and public concern, that food
is regarded as the main source of exposure and that there is a
lack of local exposure data, there is a need to conduct a study
to examine the local situation.
3.Lead is a naturally occurring toxic heavy metal which is ubiquitous in the environment
as a pollutant.It has
widespread industrial uses such as battery production, preparation
of paints and petrol, etc.Lead
can be present in food as a result of environmental pollution or
unintentional contamination during food processing, handling and
packaging.
Sources of Lead
4.Lead may enter the environment at any point during mining, smelting, processing, recycling
or disposal.Lead is
also used in solder applied to water distribution pipes and to
seam of cans used to store foods, in bottle closures for alcoholic
beverages and in ceramic glazes and crystal tableware, although
some of these usages are fading out.Its
uses would result in increase of the lead levels in soil, water
and air.In some countries where leaded petrol is still used,
the major air emission of lead is from combustion of petrol.Atmospheric lead is also a major source of lead in household
dust. [1]
5.Lead is present in soils and is transferred to food crops growing on soil.Lead
present in air may also deposit on leafy vegetables.Therefore,
cereals and vegetables may contain high level of lead.Aquatic food animals may also accumulate lead from contaminated
waters and sediments.Shellfish was reported to contain a higher level of lead
than fish since the chemical tends to bioconcentrate more
in shellfish than in fish.For
meat and meat products, since lead accumulates more in the offal,
higher level of lead would be present in offal than in the meat tissues.
[1]
6.Traditionally, lead compounds have been used for processing lime preserved egg, and
therefore a high lead level was detected in the lime preserved
egg.
Toxicity
Metabolism
7.After ingestion, the absorption rate of lead ranges from 3% to 80% whereas the typical
absorption rates of dietary lead in adults and infants are 10%
and 50%, respectively.After
absorption, lead is initially distributed to soft tissues throughout
the body via blood, and then deposited in bone.Lead
is excreted through the kidney and to a lesser extent in the bile
while non-absorbed dietary lead is excreted in the faeces.Organic lead may be metabolised to inorganic lead.The
concentration of lead in blood is commonly used as biomarker of
exposure.Other markers include bone lead concentration, urinary
concentration and dentine lead. [2]
Toxicity and Carcinogenicity
8.Lead is a classical chronic toxic chemical.Lead may cause damages to kidneys, the cardiovascular,
immune, haematopoietic, central nervous and reproductive systems.Short
term exposure to high level of lead can cause gastrointestinal
distress, anaemia, encephalopathy and death.[3]The
most critical effect of low-level exposure is retarded cognitive
and intellectual development in children.[2] Infant, young children
and the foetus are more sensitive to toxic effects of lead, especially
the damages to central nervous system. [4]
9.The International Agency for Research on Cancer (IARC) of the World Health Organization
(WHO) has evaluated the carcinogenicity of lead and lead compounds. IARC
considered that there are inadequate evidence for carcinogenicity
of both inorganic lead and organolead compounds in human, sufficient evidence for inorganic lead compounds
in experimental animals and inadequate evidence for organolead compounds in experimental animals. It classified lead and inorganic lead compounds as Group
2B agent, and organolead compounds as
Group 3 agent.[2]
Level of Safe Intake of Lead
10.The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has established a provisional
tolerable weekly intake (PTWI) of 25µg/kg bw/ week for lead.
11.PTWI is an estimate of the amount of a contaminant that can be ingested over a lifetime
without appreciable risk.An
intake above the PTWI does not automatically mean that health is
at risk.Transient excursion
above the PTWI would have no health consequences provided that
the average intake over long period is not exceeded as the emphasis
of PTWI is a lifetime exposure.
Sources of Human Exposure
12.Lead exposure can occur through food and water as well as soil and air and the relative
contributions from individual sources may depend on life-style
and socioeconomic status.It
was reported that the main sources of exposure for an adult are
food (ranged from 0.4 mg/kg bw/week to 10.1 mg/kg bw/week) and water (ranged from 0.23 mg/kg bw/week to 0.35 mg/kg bw/week).[2]
13.Airborne lead may contribute significantly to exposure, depending on various factors
such as tobacco smoking, occupation and proximity to sources, which
may vary considerably. [1][2] For smokers who smoke
20 cigarettes daily, an additional exposure of lead from cigarette
smoking was estimated to be 0.28 mg/kg bw/week.[1]
14.The main potential sources of exposure
to lead in children are food, air, water and dust or soil.It
has been reported that the dietary intakes of children (ranged
from 0.6 – 30 mg/kg bw/week) could be two to three times
that of adults.[2]
15.In other dietary exposures studies such as those conducted in the UK and
Mainland China, cereals and vegetables were found
to be the main dietary sources of lead, which contributed
31% to 40% of total dietary exposure for cereals,
and 23% to 35% of total dietary exposure for vegetables.
[5][6]
SCOPE OF STUDY
16.To estimate the dietary exposure to lead, this study covered six major food groups,
namely (i) cereals and cereal products,
(ii) vegetables, (iii) fruits, (iv) meat, poultry, egg and their
products, (v) seafood, (vi) milk and dairy products.The
selection was based on the occurrence of lead in those food groups
and the consumption patterns.
17.As leafy vegetable was reported to be contaminated with lead through adherence of the
lead present in air, the study also explored the measures which
can be taken at the consumer level.
METHODOLOGY
Dietary Exposure
Food Consumption Data
18.The food consumption data in this report were extracted from the Food Consumption Survey
conducted in local secondary school students in 2000 by FEHD.In
the survey, a stratified three-stage sampling plan was used, with
a sampling frame of 472 secondary schools and more than 380,000
students, covering almost all the local secondary schools.A
total of 967 students from 27 schools participated in the survey
yielding a response rate of 77% at the school level and 96% at
the student level.The mean weight of the participated students was 52.0
kg. [7]
Sampling Plan
19.Food samples were taken from the
local market according to the six food groups as mentioned above.Food items were selected so as to match those in the
Food Consumption Survey as well as those with likely occurrence
of lead.Three samples
of each food item from different sources were taken for analysis.
Laboratory Analysis
20.Laboratory analysis was done by
the Food Research Laboratory (FRL) of FEHD. The analysis was conducted mainly in the form of “composite
sample”.Samples
collected randomly from the three different sources were mixed
and homogenised. The composite sample was then digested with concentrated
acid, followed by determination of lead using inductively coupled
plasma mass spectrometry (ICP-MS).The
limit of detection (LOD) was 0.6 mg/kg.
21.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.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 lead, usually follows
a log-normal distribution, assigning a value of 1/2 LOD to all
non-detected levels is considered as a conservative approach for
food groups in which the majority of food items have levels below
the LOD.
Dietary Exposures to Lead
22.Dietary exposure from individual food item was obtained by combining the consumption
data and the concentration of individual food items.Daily dietary exposure was multiplied by seven to obtain
a weekly exposure level.Total
exposure for each student was obtained by summing exposures from
all food items.The
mean and 95th percentile of the weekly exposure levels
were used to represent the average dietary exposure and the exposure
for high consumers respectively.
23.The estimated weekly exposure levels were then compared with the PTWI as established
by JECFA.
Reduction of Level of Lead in Leafy Vegetables
Sampling and Treatment of Samples
24.Two varieties of leafy vegetables were selected based on the findings of the initial
results.Three samples
of each variety from different sources were taken for analysis.The six leafy vegetables samples were subject to soaking
in water for 30 minutes, washed in running water for 1 minute,
and then cooked in boiling water for 3 minutes.The levels of lead in the samples were determined at
different stages: (i) raw and as sold,
(ii) after soaking and washing, and (iii) after boiling.
25.Another sets of bakchoi and flowering
cabbages samples were also obtained and they were subject to cooking
in boiling water without prior washing or soaking.The levels of lead were determined (i) raw and as sold and (ii) after boiling.
RESULTS
Food Consumption Data
26.Food consumption data for the six food groups are given in Table 1.
Table 1:Food
Consumption for Secondary School Students
Food groups
Mean consumption
(g/day)
Cereal and cereal products
478.0
Vegetables
295.3
Fruits
309.1
Meat, poultry, egg and their products
189.4
Seafood
122.4
Milk and dairy products
143.2
Concentration of Lead in Food
27.A total of 345 food samples were taken and combined into composite samples and 117 analyses
were done.The results
are given in Table 2.
Table 2:Number
of Samples Taken and Median Concentrations for Lead in Six
Food Groups
Food groups
Number of samples
% of samples below LOD
Median concentration (mg/kg)
Cereal and cereal products
45
93.3
<LOD*
Vegetables
81
3.7
14.2
Fruits
21
71.4
<LOD *
Meat, poultry, egg and their products
99
63.6
<LOD *
Seafood
84
10.7
7.6
Milk and dairy products
15
100.0
<LOD *
(* a value of 1/2 LOD was
assigned, i.e. 0.3mg/kg)
28.Lead was detected in most of the samples of the two food groups “vegetables” and “seafood”.For
the other food groups except “milk and dairy products”,
lead was detected at low levels in most of samples.As
for “milk and dairy products”, lead was not detected
in all the samples.The distribution curves of lead concentration in the
six food groups are given in Annex.
Dietary Exposures to Lead
AverageSecondary School Students
29.The dietary exposure to lead for average secondary school students was estimated to
be 1.98
