Current
approaches may be underestimating the risks posed to children by toxic
metals that they are exposed to while playing outdoors, according to
Japanese researchers. The researchers used a new approach that they
developed for their study of how much dirt and associated metals adheres
to children’s hands. They found that lead, chromium, zinc and other
heavy metals adhere more to smaller soil particles than to the larger
particles typically employed in soil exposure studies. That finding
suggests using the researcher’s new approach going forward to better
assess children’s soil-related risks.
Children’s
ingestion of contaminants through soil has long been a concern of the
U.S. Environmental Protection Agency and its counterparts in other
nations, so the study by researchers in Japan adds new insights into
better protections for children and the risks posed by soil-related
exposures.
As the study explains, soil ingestion is one of the
most important pathways through which children are exposed to toxic
substances. Children have higher exposure rates from soil than adults
because of their hand-to-mouth behavior. As they play outside in dirt
mounds and playgrounds, there is a risk that children will ingest soil
particles and heavy metals which may have been underestimated by
researchers to date.
For the study, researchers collected soil
samples from 58 playgrounds located in Kyoto City in Japan and used a
standard 2-millimeter (mm), or 2,000 microns (µm), “sieving system” to
measure the quantity of heavy metals in the soil. But, according to the
paper by Maiko Ikegami of Kyoto University’s Research Reactor Institute
and colleagues, using “2-mm sieving in preparation for measuring heavy
metal content caused underestimation of the risk of direct soil intake.”
The paper, “Effect of Particle Size on Risk Assessment of Direct Soil
Ingestion and Metals Adhered to Children’s Hands at Playgrounds,” has
been posted electronically in Risk Analysis: An International Journal,
published by the Society for Risk Analysis. Dr. Ikegami’s collaborators
include Minoru Yoneda, Takashi Tsuji, Osamu Bannai, and Shinsuke
Morisawa.
As part of their study, the researchers adopted a
hydrochloric acid (HC1) extraction method, which is thought to be
capable of measuring the approximate quantity of heavy metals that can
be absorbed by the human body. It is used for extracting heavy metals to
evaluate the risk of direct soil ingestion. A basic expectation of
current soil risk analysis is that soil particles that adhere to the
body or are ingested directly from the air will be smaller than 2 mm in
size. Some studies have shown, however, that the concentration of metals
in soil increases with smaller particle sizes, such as particles less
than 45 µm.
For the study, 69 children in
a Kyoto City nursery school were selected “to measure the amounts of
metal adhered to their hands on a clear day.” After playing outside for
30 minutes, the children had their hands—including fingers, thumb and
palm—wiped with cotton pads wetted with purified water. The amount of
soil adhered to the children’s hands was estimated from the quantity of
metals on their hands.
In conducting their evaluation of
particle-size distribution of soil adhered to human hands, the
researchers first removed large soil particles that were not obviously
adhered to hands from the surface area. After some students' hands were
washed and dried, they were rubbed on the surface soil for 30 seconds,
moving in a 60-centimeter-wide horizontal direction at 30 times per
minute. Soil was removed from the students' hands by tapping the hands
without washing until soil particles no longer fell off. Their hands
were then washed in 300 milliliters of ultrapure water, creating a
liquid sample whose particle-size distribution was then measured.
“To
accurately evaluate the risk, it is important to understand what size
of the soil particles children ingest directly,” according to the paper.
The study “found that smaller particles had a tendency to contain more
heavy metals than bigger particles.” According to the authors, the HC1
extraction method using samples of soil smaller than 2 mm “may
underestimate the risk of direct soil ingestion if the soil size
ingested is usually much smaller than 2 mm.” Researchers measured the
particle-size distribution of soil adhered to three students’ hands,
producing a particle-size distribution for all samples of 0.22–313.08
µm. Results indicated that approximately 90 percent of soil particles
remaining on the hands were less than 100 µm, suggesting that
laboratories and researchers should transition to the method used by the
authors in order to avoid underestimating children’s risks.
In
general, the researchers conclude that more careful methods for
assessing soil ingestion are necessary to protect children who ingest
soil with metals while playing on dirt playgrounds, near hazardous waste
sites, or in areas with heavy vehicle traffic. Also of concern are
so-called “pica children,” those with the unusual habit of deliberately
ingesting large amounts of soil and the associated contaminants.
Story Source:
http://www.sciencedaily.com
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