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Date: Friday, September 6, 1996 FOR IMMEDIATE RELEASE NCI Press Office (301) 496-6641
Contamination of drinking water with nitrate, a chemical in fertilizers, may be associated with an increased risk of non-Hodgkin lymphoma (NHL), particularly in agricultural areas, a National Cancer Institute study suggests.
In a study published in the September issue of the journal Epidemiology,*
scientists from NCI, the University of Nebraska Medical Center
in Omaha, and Johns Hopkins University in Baltimore assessed the
average amount of nitrate consumed daily in tap water by Nebraska
residents diagnosed with NHL, a cancer of the lymphatic system,
and by a control group of persons without the disease who lived
in the same area. Both groups used public water supplies.
The more nitrate they consumed in their water, the greater was
their probability of developing NHL. Persons with NHL were twice
as likely to be in the group that consumed the highest levels
of nitrate as those without the cancer.
This is one of the first epidemiologic studies to suggest a link between drinking-water
nitrate and non-Hodgkin lymphoma risk said
Mary H. Ward, Ph.D., of NCI, the study
lead author. The findings
deserve further evaluation because nitrate is a common contaminant
of ground water in many areas of the country.
However, it is uncertain whether the findings truly reflect the
effect of nitrate, she added. An alternate possibility is that
nitrate exposure is simply a surrogate or A marker
variable that is correlated with another NHL risk factor that
was not directly measured in the study. (The researchers did assess
occupational pesticide exposure, however, and determined that
it could not account for their results.) NCI is conducting additional
research that will help determine with greater certainty whether
the association is real, and if so, how large it is.
Since 1973, incidence of NHL in the United States has increased
about 75 percent -- one of the largest increases among major cancer
sites. An estimated 52,700 Americans will be diagnosed with the
disease in 1996, and 23,300 will die from it. NCI has estimated
that about 1 in 52 men and 1 in 61 women in the United States
will be diagnosed with NHL during their lifetime, and that slightly
fewer than 1 in 100 of both sexes will die of the disease.
Part of the increase in NHL incidence in recent years is a result
of the AIDS epidemic: NHL is 60 times more common among AIDS patients
than in the general U.S. population. Herbicides and insecticides
have been linked to risk for NHL in studies of farmers, pesticide
applicators, and other occupational groups exposed to high levels
of these chemicals.
Nitrate forms N-nitroso compounds, many of which
are known animal carcinogens. Biochemical studies in humans
have shown that nitrate in water combines with amino acids to
form these compounds.
The increase in mortality from NHL has been greatest in rural
areas, a fact that could only partly be explained by pesticide
exposures in agricultural workers. The new findings on drinking-water
nitrate among Nebraska residents who were not farmers suggest
that nitrate, or some other exposure that is correlated with nitrate
exposure, may be another NHL risk factor. Earlier studies in Nebraska
and China suggested that NHL and leukemia might be linked to nitrate
intake, while a Danish study found no link between NHL and drinking-water
nitrate.
Nitrate levels in ground and surface waters of agricultural regions
have increased over the past 40 years as a result of increases
in the use of nitrogen fertilizers. Nitrate contamination occurs
in geographic patterns related to the amount of nitrogen contributed
by fertilizers, manure, and airborne sources such as automobile
and industrial emissions, and to soil drainage characteristics.
Areas with well-drained soils and high nitrogen inputs have the
highest nitrate levels in the water supply. In particular, large
areas of the Midwestern corn
belt states have nitrate
levels above natural levels. The U.S. Environmental Protection
Agency's regulatory
limit for nitrate is 10 mg per liter of drinking water.
One advantage of the new study -- part of a larger case-control
study that also examined associations between NHL and the herbicide
2,4-D, along with dietary and other possible risk factors -- is
that the researchers calculated nitrate consumption levels for
each person rather than simply comparing cancer rates in large
populations with differing nitrate levels in their water supplies.
To do this, they determined from public records how much nitrate
was in each person's
community water source starting in 1947, and asked subjects how
much they typically drank each day of tap water, coffee, and other
beverages made with tap water.
The researchers estimated drinking-water nitrate consumption for
90 women and 66 men diagnosed with NHL between 1983 and 1986 who
used community water sources, and for a control group of 276 women
and 251 men in the same eastern Nebraska counties who also used
community water. Those with NHL were twice as likely to be in
the group with the highest drinking-water nitrate consumption
levels (an average of 6.3 mg per day or more over their adult
life) as those without the cancer. Persons exposed to drinking-water
nitrate levels above the EPA limit of 10 mg/liter for one year
or longer -- a group that made up 21 percent of all persons in
the study -- had a 50 percent higher risk than those having no
exposure above this level.
A separate analysis of NHL cases and controls who used private
well water found no association between NHL incidence and nitrate
levels in well water. However, unlike community water sources,
no long-term records of nitrate levels were available for private
wells. Levels in wells were measured only once, at the time of
the study, making it more difficult to determine cancer risk from
nitrate exposure for persons using private water sources. Twenty
percent of private wells exceeded the EPA recommended level. In
general, persons using private wells are more likely to drink
water containing high nitrate concentrations than persons using
public water supplies, because of the simpler construction and
shallower depth of private wells.
Nitrate intake from dietary sources was also estimated for each
person in the study. Higher dietary nitrate consumption -- mostly
from vegetables including spinach, lettuce, and beets -- was associated
with lower risk for NHL. This apparently contradictory finding
may be explained by the anticarcinogenic effects of vegetable
components such as vitamin C and carotenes (vitamin A-related
compounds).
NCI is further investigating the link between drinking-water nitrate
and NHL in a series of case-control studies among residents of
Iowa and Minnesota, other agricultural states that have areas
of elevated groundwater nitrate levels. Nitrate consumption is
also being studied as a possible risk factor for stomach cancer,
a disease that is less common in the United States than in Asia
and some other parts of the world.
The Cancer Information Service provides a nationwide telephone
service for cancer patients and their families, the public, and
health care professionals. The toll-free number is CANCER
(1-800-422-6237);
services provided in English and Spanish. People with TTY equipment
may call 1-800-332B-8615.
This document is available through the NCI's CancerNet services
on the Web (cancernet.nci.nih.gov) and through Cancer Fax
(dial 301-402-5874 from the handset on your fax machine).