International Journal of Risk & Safety in Medicine 9 (1996) 33-40 IOS Press
Michael Sammut
Department of Toxicology, Malta Medical School, Gwardamangia, Malta
Tel.: 0356 223035/221 019/239783; Fax: 0356 235638
Charles Savona-Ventura (Corresponding Author)
Department of Obstetrics & Gynaecology, St. Luke's Teaching Hospital,
Gwardamangia, Malta
Tel.: 0356241251/223035
Abstract. The Maltese population has been repeatedly shown to have high mean blood lead levels irrespective of age. This study confirms that these higher levels also occur in Maltese children. A definite correlation is shown between high blood lead levels and the environment of the subjects studied, being statistically higher in individuals living on the urbanized larger island. A statistical correlation has been shown between hand-wipe specimens taken from children playing in a playground and the traffic density of the area. A similar statistical correlation between playground, street and household dust to traffic density has also been identified.
Keywords: Lead, traffic-density, environment
1. Introduction
Trace elements have an increasing impact on clinical and community medicine.
The effects of many of these elements are ill-understood, while their interactions
with other major or minor nutrients have still to be fully elucidated.
The toxic effects of acute lead poisoning are well documented. Chronic
exposure to subtoxic lead levels is increasingly being shown to also be
harmful, particularly during periods of accelerated' growth. The Maltese
population has been repeatedly shown to have higher blood levels than the
reference values proposed by a directive of the European Community. These
high blood levels have been reported to affect all segments of the population
irrespective of sex and age and have been also demonstrated in the neonate
[1-3].
One suggested source of lead in the Maltese ecosystem is the high population
density with the oncomitant high automobile density. The Maltese Islands
are a small group of islands with a total area of only 313 km2.
The number of registered cars in 1990 amounted to 182254, giving an automobile
density of 582 per km2. The consumption
of unleaded petrol is only about a sixth of that of leaded petrol, and
about 20 tons of lead are released in the environment annually [4]. The
present study attempts to correlate blood lead levels and lead levels in
playground, street and household dust with traffic density, and the possible
significance of dust to chronic lead exposure in young children.
2. Material and methods
Blood lead levels of a total of 103 randomly chosen school children,
aged 7-9 years, were assessed. The children were subdivided into two broad
groups: the first (n = 68) living on Malta, while the remainder (n = 35)
living in Gozo. The latter island is less densely populated and may be
considered overall rural when compared to the larger island. Three playgrounds
in Malta exposed to a varied traffic density were identified. The traffic
density for these three areas has been assessed by direct counting to be
2479, 8400 and 23 850 cars per day, respectively [5]. Ten dust samples
were collected from three playgrounds in the months of August and December
when the average rainfall is 2.5 and 111.3 mm, respectively. Wet handwipe
samples were also taken during August from ten children in each playground
aged up to 12 years who had been playing for at least half an hour in the
field. Furthermore, street and household dust was collected in September
(mean rainfall 47.0 mm) from 41 localities in Malta. Household dust deposition
was further assessed over a two-week period by leaving two petri-dishes
(8.0 cm diameter) filled with 8 ml of 65% glycerin in water at a height
about 2.5 m from the ground.
The blood lead levels (PbB) were assayed by atomic absorption spectrophotometry
electrothermal atomization (AAS-ETA) using a modified Fernandez method.
The dust samples were first passed through a 500 um sieve to standardize
particle size. The samples were then dried at 90oC for 24 hours
after which 0.5 g were dissolved in 10 cm3 of Analar concentrated
nitric acid. After cold digestion, the tubes were heated to 900C for 24
hours. After filtering, the solutions were made up to 100 cm3 volume with
de-ionized water. The street and household dust samples were treated in
a similar fashion dissolving them in 20 ml of nitric acid and subsequently,
after concentration by boiling, made up to 25 ml with de-ionized water.
The estimation of lead in deposited dust samples was performed by concentrating
the solution in the petri dish to about S ml, this being re-made up to
10 ml with de-ionized water. Flame atomic absorption spectrophotometry
(Varian AAS model 1250) was used to estimate the level of lead in the dust.
The wet wipe samples were similarly placed in 10 cm3 of nitric
acid and heated at 90oC for 24 hours. After filtering, the solutions
were made up to 100 cm3 with distilled water. A flameless atomic
absorption spectrophotometric technique (Perkin Elmer AAS model 2100 +
HGA 700) was used to assess lead levels. Statistical analyses were performed
using the Student's t-test and single linear regression.
3. Results
The traffic density showed a significant correlation to the blood lead
levels in children. Thus the mean ± sd blood lead levels in Maltese
children was at 241.5 ± 72.5 ug/l, significantly higher than the
mean levels noted in Gozitan children living in a more rural environment
(194.4 ± 77.9 ug/l; p < 0.001). The distribution pattern of blood
lead levels also demonstrated a shift towards higher levels on the main
island Malta. In Malta a total of 46 children (67.6%) had blood lead levels
greater than 200 ugIl, while 14 Gozitan children (40%) had such elevated
levels (Fig. 1).
There appeared to be a definite statistical correlation between lead
levels and traffic density in the region of the three playing fields (Table
1, Fig. 2). Thus the mean dust lead level in
summer rose from 0.0981 ± 0.049 mg/g in the low traffic density
area to 0.325 ± 0.097 mg/g in the high traffic density playground
(r = 0.78, p < 0.001). A similar pattern is noted during the winter
months (r = 0.84, p < 0.001). Though there did appear to be a slight
reduction in lead levels during the winter in two of the sites studied,
this difference was not statistically significant. in contrast, the nign
traffic playground showed a non-statistically significant rise in lead
levels during the winter months.
There appeared to be a close relationship between dust lead levels
and the lead levels on the hands of children in the playing fields. Thus
a definite significant rise of lead swab levels (r = 0.74, p < 0.001)
from 10.09 1 9.09 j£g/swab in the low traffic density field to 56.84±126.96
ug/swab in the high traffic density field was observed (Table
1, Fig. 2). A corresponding relationship (r
= 0.64, p < 0.001) between swab lead levels and dust lead levels could
also be demonstrated (Fig. 3).
The importance of traffic density towards the contribution of lead
in street dust was further confirmed when street dust lead levels from
various localities were assessed. A statistically significant relationship
(r = 0.84, p < 0.001) between traffic density to lead levels in street
dust could be demonstrated (Fig. 4). When the various
localities were classified on the basis of traffic density into urban,
suburban and rural categories, a statistically significant rise in the
lead levels of all the dust specimens could be noted through the various
categories, with street dust lead levels rising from the 188±235
ug/g level in rural sites to 1473±1115 ug/g in urban areas. House
dust and deposited dust lead levels followed similar trends (Table
2).
The transfer of street dust to households appears to be related (r
= 0.64, p < 0.001) to the levels of street dust (Fig.
5), being apparently dependent on atmospheric transfer and deposition.
Thus the relationship between lead levels in household dust and traffic
density followed a similar pattern (r = 0.65, p < 0.001) (Fig. 4), while
a relationship ( r = 0.72, p < 0.001) could also be observed between
household dust lead levels and deposited dust lead levels (Fig.
6).
4. Discussion
Lead is ubiquitous in the environment and consequently it can reach
the human organism through a number of pathways. There are a number
of sources which may introduce lead in the environment. Common entry
portals include industries, lead paint and vehicular emissions. The
latter is an important contributor to lead in the Maltese ecosystem in
view of the high traffic density which pertains to the Islands. The Maltese
population has been shown to have remarkably high blood lead levels when
compared to the reference levels proposed by the EEC and the CDC (200 ug
Pb/l). Thus the mean blood lead levels in the Maltese was 307 ug Pb/l.
A follow-up study two years later reported a slight drop in the mean blood
lead levels, but was still elevated at 243 ug Pb/l. The levels were elevated
in all segments of the population, including neonates [1-3]. The present
study suggests that blood lead levels are similarly elevated in Maltese
children aged 7-9 years with a mean of 241.5 ug Pb/l in Malta and 194.4
ug Pb/l in Gozo. The latter island has a more rural environment promoting
a lower traffic density, thus accounting for the statistically lower blood
lead levels observed. About two-thirds of Maltese children and two-fifths
of Gozitan children have blood lead levels which exceed the critical value
recommended by the Centers of Disease Control, above which a complete medical
evaluation needs to be conducted. The CDC also recommends that community
prevention activities should be initiated for blood levels above 100 ug/l
[6]. The blood lead levels in the rural community seem higher than would
be expected from petrol lead exposure alone, thus suggesting that the source
of lead in the environment is multifactorial.
Investigations attempting to identify a particular lead source that
contributed to these elevated lead levels in the Maltese population has
proved to be difficult. Food analyses have shown that lead levels in Maltese
foodstuffs, except of potatoes, flour, carrots and pasta, compared well
to those from similar food items from Mexico, Belgium and Sweden. Soil
sample analyses have further demonstrated that soil lead levels rose in
soil samples obtained close to a road. The soil lead levels ranged from
29.0 - 367 ug/g dry material. The airbome lead content was found to be
about 1 ug/m3 in areas with traffic congestion.
This airborne lead has been estimated to result in an increase of 1.5-3.0
ug Pb/dl of blood [3]. The present study shows definite relationships between
traffic density and dust lead levels, including street dust, household
dust and playground dust, thus demonstrating the importance of vehicular
emissions to environmental lead. Car ownership in Malta is amongst the
highest in Europe, estimated at an average of 1.7 cars per household. With
the small surface area of the Islands, this car ownership level results
in a very high traffic density. There is at present no legal restriction
on the quantity of lead in petrol sold on the Islands, with recent estimates
reporting a value of 0.32 g Pb/l. Leaded petrol consumption in 1994 reached
a value of 63647 m3, suggesting a total annual lead emission
of about 20.6 tons. The consumption of unleaded petrol remains low 10242
m3 [4]. Lead compounds derived from vehicular emissions are
mostly inorganic and consequently exist in a particulate form. Lead, in
the form of tetra-methyl lead or tetra-ethyl lead, is added to petrol to
increase its octane rating to reduce engine knocking. Ethylene dibromide
and ethylene dichloride are further added to petrol to remove the lead
from the engine cylinders. The predominant lead product exhausted from
automobiles is lead halides, which undergo chemical changes quite rapidly
with other exhaust gases and in the atmosphere [7].
Vehicular lead sources may enter the human organism through inhalation
and/or ingestion. Inhaled lead is taken up more efficiently than through
ingestion. The uptake of lead by inhalation is dependent on the lead concentration
in the air, particle size and the breathing and ventilation rates. In the
situation of air lead levels of 1 ug/m3 described for the Maltese ecosystem,
an adult is estimated to take 15 ug Pb/day of which 50% would be deposited
in the lungs. Ninety per cent of the deposited lead is absorbed and equilibrates
between the circulation and the tissues. In the Maltese circumstances therefore,
an adult living in areas of traffic congestion would absorb about 7.5 ug
Pb/day of which 3.75 ug Pb/day remains in the circulation [8,9]. Absorption
of lead through the gastrointestinal tract is a relatively inefficient
process. A number of conditions influence the rate of absorption of lead,
particularly the chemical form in which it is ingested, the type of diet,
and the individual's physiology. It has been estimated that the absorption
of lead from the gastrointestinal tract into the bloodstream averages 10%
in adults and 40% in infants [8-10]. Vehicular lead emissions are sparingly
soluble in water, but have been found to be 90% soluble in hydrochloric
acid at a pH 1, stimulating the chemical conditions inside the stomach.
The soluble lead forms are more efficiently absorbed by the gastrointestinal
tract [11].
Dust ingestion seems to be an unlikely pathway in adults. It has however
been shown to be a significant source in children [9-12]. Investigations
have revealed that dust may account for half the total daily intake of
lead by a young child amounting to an average value of 50-100 ug Pb/day
[9,13]. Dust can be ingested indirectly by eating sweets or sucking toys
while playing at home or in the street. The child can transfer up to 50
mg of dust from his hands to a sweet after 30 minutes activity in a playground
[11]. The present study has demonstrated the high lead levels in dust from
playgrounds in high traffic density areas and the relationship of dust
lead levels to child hand-wipe lead levels. The risks of lead intake through
ingestion of dust extends also in the home since housedust in urban areas
has also been shown to have significantly higher lead levels when compared
to rural areas. The increased transfer of lead from household, street and
playground dust in areas of high traffic density has been demonstrated
by the higher blood lead levels noted in children living in Malta compared
to those living on the more rural island.
While vehicular emissions contribute substantially to the lead levels
in soil, which levels are in-versely proportional to the distance from
the road [2,13]; the organic matter in soil together with the high alkalinity
of Maltese calcareous soils immobilizes the lead decreasing its availability
to plants [13]. Thus fruit and vegetables, provided they are well washed
from adhering soil and dust, are unlikely to be direct sources of lead
ingestion. On the other hand, seafood may be an important source of ingestion
of vehicular-originating lead in the Maltese circumstances. The level of
lead in sea sediments in the vicinity of the capital city was found at
118 u/g to be markedly higher to levels of other Mediterranean inshore
sediment exposed to urban and industrial pollution (Thermaikos Gulf: 71
ug/g; Gulf of Venice: 45 ug/g). Certain edible shellfish, such as the Date
Mussel and the Warty Venus, collected from Maltese waters have also been
found to carry high lead levels. The main source of this high sediment
lead is most likely to be car traffic rain washoff [14].
Vehicular emission is a definite contributor of lead in the Maltese
environment resulting in the reported high blood lead levels of this population.
Serious efforts to reduce the amount of leadened petrol and promote lead-free
petrol must be taken by the authorities. There is evidence that the environmental
benefits of lead-free petrol are being increasingly appreciated by the
Maltese, resulting in an increase in lead-free petrol demands from 5281
m3 in 1992 to 10242 m3 in 1994. In spite of this
94% increase in lead-free petrol demands, the leadened petrol at 63 647
m3 in 1994 remains the major source of automobile fuel demanded
[4].
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