Scientists have a new tool to help them unravel the mysteries of the toxicity of dioxin. The development of the aryl hydrocarbon receptor-deficient mouse was reported by Frank Gonzalez and colleagues of the National Cancer Institute this May in Science.
The controversy about the health effects of dioxin partly involves questions about how its toxicity is mediated. Most scientists agree that dioxin exerts its effects by binding to the aryl hydrocarbon receptor (AhR). What is not clear is how this binding relates to the particular cell types that are affected.
The AhR-deficient mouse will help answer questions about the mechanism of dioxin and similar compounds such as benzo[a]pyrene, PCBs, and PBBs. Gonzalez and his team produced the mouse by "knocking out" the gene that encodes the AhR. It is known that the AhR detoxifies poisons, but the NCI researchers found evidence that it has other important functions as well.
Half of the AhR-deficient mice die within a week after birth due to a lack of lymphocytes that leaves them susceptible to opportunistic infections. The mice that do survive have massive liver scars and only slowly build up the normal number of lymphocytes. At 10 weeks of age, the animals begin to lose the lymphocytes they built up and eventually become sick due to an incompetent immune system and liver problems. The livers of these mice are 50% smaller than normal, and they have bile duct fibrosis.
The AhR is obviously vital to immune function and liver health. But the depression of the immune system of AhR-deficient mice is a puzzle because the thymus, where T-lymphocytes mature, is normal in these animals. Gonzalez and co-workers hypothesize that AhR-deficient mice may lack a specific lymphoid population or have a systemic defect in the ability of lymphocytes to reside in the peripheral immune system (which includes the reticuloendothelial system, of which the liver is a component). Loss of the AhR may affect thymic processes or affect the migration of cells from the bone marrow (where precursor lymphocytes originate) to the thymus or to peripheral lymphoid organs. Alternatively, the normal life span of peripheral lymphocytes may be shortened in these animals.
Previous research has shown that the AhR may also play a role in brain development. Levels of AhR are high in the fetal neural tube, which gives rise to the central nervous system. Levels decrease after birth. In addition, AhR is found in the kidneys, lungs, and hearts of adults. It is hoped that the AhR-deficient mouse will help clarify the functions of the receptor. However, Gonzalez cautions that experiments with these mice may be difficult because of their poor health. Further genetic engineering may have to be done to turn on the AhR gene only in liver tissue so that the mice will be hardy enough to withstand testing.
Liver trouble. Accumulation of collagen (blue) around the liver bile duct of a 30-day-old AhR knockout mouse (right) shows the beginning of fibrosis. (Left) Liver bile duct of a normal mouse. Jerrold Ward/NCI
In the wake of a report that healthy people can be infected with infinitesimal exposures to Cryptosporidium comes the Natural Resources Defense Council's assertion that at least 45 million Americans are at risk of imbibing the diarrhea-causing protozoan in what appears to be clean drinking water.
At the University of Texas in Houston, infectious disease expert Herbert L. DuPont gave 112 healthy volunteers preparations containing between 30 and 1 million Cryptosporidium oocysts (the form in which the microbes are found in water). Monitoring enteric symptoms and analyzing stool samples for excreted oocysts, DuPont found that, for the strain he used, the median infective dose was only 132 oocysts. The study suggested the size of the dose did not affect the microbes' incubation period or the severity of the infection.
Drinkable danger. New research shows that relatively small numbers of Cryptosporidium oocysts are enough to cause potentially severe infections.Frank W. Schaefer/EPA
Just how many people may become infected from their drinking water is impossible to estimate. The NRDC figure of 45 million is based on a survey mailed to 100 of the nation's 61,000 water suppliers, says the organization's president, Eric Olson. The systems who responded serve only a fraction of Americans.
Cryptosporidia give rise to dormant oocysts that remain viable for months in sewage, runoff from feedlots, or groundwater until they find a new host. Unlike other waterborne organisms, the oocysts are neither killed by chlorine nor screened by standard filters, says DuPont. When one member of a household is infected, secondary spread can occur.
Once thought to infect only animals, especially young cattle, Cryptosporidium came to the attention of health authorities in the 1980s, when it was found to cause life-threatening, chronic diarrhea in AIDS patients. In immunocompetent hosts the microbe typically causes a day or two of discomfort, with symptoms including nausea, vomiting, diarrhea, and cramps. Most sufferers need only oral rehydration until they recover.
"Crypto is a nasty nuisance, but not a large-scale public health menace," says DuPont, now at Baylor College of Medicine. Serologic evidence of past infection is found in 15% or more of Americans and nearly 100% of people in tropical areas with poor sanitation. "Here [in the United States] it probably causes many outbreaks of diarrhea in children at day care centers," Dupont said.
But to anyone with impaired immunity, the normally self-limiting illness can be fatal. At risk are patients taking immunosuppressive drugs (to treat cancer or prevent organ transplant rejection), anyone taking steroids, and older people.
Recent guidelines from the CDC and the EPA say waterborne Cryptosporidium can be eliminated by boiling water for one minute or by using a home filtering device to screen particles less than one micron in diameter. Suitable are reverse-osmosis devices, those certified for "cyst removal" by the National Sanitary Foundation, and those labeled as "absolute" for one micrometer.
Point-of-use treatment doesn't satisfy NRDC, however. "Water suppliers shouldn't throw this problem in the lap of those at risk--the sick, the poor, and the elderly. Is it right to ask society's weakest members to boil their drinking water or buy a purifier?" Olson asks.
And expensive bottled water can't be assumed to be oocyte-free unless it's been distilled, properly filtered, or came from a protected spring or other pristine source, something consumers are relatively powerless to determine.
The cost of Cryptosporidium-free tap water will vary according to the size of the system, says environmental engineer Stig Regli of the EPA. For an existing major municipal system to add an additional filtering step, or more effective but costlier ozone disinfection would add $10 to $15 per year to a residential water bill, while patrons of small public or private systems might have to pay $100 or more per year, says Regli.
The CDC/EPA guidelines urge individuals who may be worried about Cryptosporidium to ask their health care provider about appropriate risk-reducing measures. Current data don't justify telling immunocompromised people to boil or avoid drinking tap water unless there's an outbreak, guidelines say, which warn that narrowly focusing on Cryptosporidium (or other single health risk) could draw attention from other potential opportunistic infections.
Even the most motivated individual will find absolute safety from Cryptosporidium hard to attain. The CDC/EPA guidelines suggest, "Individuals who contact bottlers or filter manufacturers for information should request data supporting claims that a brand of bottled water or filter can meet the above criteria." No agency lists the brands of either safe bottled water or effective home water filter systems, though a list of filters meeting CDC/EPA criteria is available from National Science Foundation (1-800-NSF-8010).
This fall, water systems can enter a voluntary quality control program sponsored by the EPA and the American Waterworks Association, which will certify that they are doing everything feasible to keep the water safe. By participating, systems may shield themselves somewhat from liability if there's a Cryptosporidium outbreak similar to the one in 1993 in Milwaukee which incapacitated thousands and led to the deaths of several immunocompromised individuals. "Everybody wants to avoid another Milwaukee," says Regli.
But until and unless water systems eliminate Cryptosporidium, DuPont urges anyone with compromised immune function, including anyone over the age of 80, to boil their drinking water, invest in a certified filter, or seek out a reliable brand of bottled water.
Many people know hemp (Cannabis sativa), which contains the psychoactive drug tetrahydrocannabinol, as the marijuana plant. But for centuries the Asian herb has been used to make rope or cord, especially large-diameter ropes for ships. Now attention is turning to the use of hemp to make paper.
Hemportunities? Researchers, manufacturers, and government officials are all looking at possible uses for hemp including paper, clothing, and other products. Joseph Tart
"It's one of the best fiber sources [for paper] around," says John Ralph, professor of forestry at the University of Wisconsin-Madison. Hemp has long fibers which increase the strength of paper made from it.
Because cultivating marijuana plants is illegal, hemp cannot be grown in the United States. Hemp is widely grown in Hungary and China, and several firms import hemp paper into the United States. Etienne Fontan, a sales manager of the Virginia-based firm Ecolution, touts hemp's environmental advantages. Fontan says that producing hemp paper is more environmentally benign than producing paper from wood. Hemp doesn't require the chlorine bleach and acids used to make paper from wood pulp.
In April of this year, Tree Free EcoPaper of Portland, Oregon, made what it said is the first commercial U.S.-produced hemp-containing paper in 40 years. The paper, made at the company's Massachusetts paper mill, is composed of 10% hemp grown in Europe, plus other nonwoody and recycled materials. According to firm President Paul Stanford, the paper is a high-quality bond paper. Stanford said EcoPaper also plans to use imported hemp to make a lower-grade paper for copiers.
Hemp can be combined with other materials to add strength. Roger Rowell, a researcher at the U.S. Forest Products Laboratory in Madison, Wisconsin, said hemp can be added to plastic products, such as fan blades, to help stiffen them. Hemp fibers are also used in clothing.
The idea of using hemp is being taken seriously at governmental levels as well. Erwin Scholts, in charge of developing and diversifying agriculture at the Wisconsin Agriculture Department, called a conference last spring to explore the commercial possibilities of hemp. And the governor of Kentucky set up a task force with a similar goal.
But the law banning the growth of hemp is a major barrier to commercialization. "It's killing it," says Rowell about the law's impact on any potential hemp industry. Rowell gave up plans to grow the plant for research purposes when he learned about the strict security and record-keeping measures he would have to take.
Scholts said that it may be possible to develop hemp commercially by breeding in genetic markers, such as color, that would identify the crop. And scientists say that hemp lacking the psychoactive ingredient tetrahydrocannabinol can easily be bred.
The governor's task force in Kentucky, which studied hemp's commercial possibilities over six months, concluded in June that the crop had no value for the state's farmers. According to Scott Smith, a University of Kentucky agronomy professor who worked with that task force, hemp's gross return would be $200 an acre. "That's not enough to interest many farmers," he said. But Gale Glenn, a farmer on the task force, disagreed, saying the question of profit hasn't been completely answered.
Hemp isn't the only plant that can be used to make paper and fiber-containing products. Jute, kenaf, and flax are also used, as is bagasse, the waste from sugar cane.
And the cost of hemp may simply be too high, compared to those plants and to wood, to make it very attractive in the United States, according to Rowell. For example, kenaf, which is used in specialty papers, sells for $.15 a pound, he said, and wood fiber sells at $.03-.05 a pound. Rowell estimates that hemp would sell for about $.50 and $.75 a pound.
William Lopatin, a project manager at the Ohio Hempery, which sells hemp-containing paper, acknowledged that hemp is expensive and that the price of other grasses would have to "go up dramatically" for hemp to be economical. Others say that hemp prices would decrease as markets for it increase.
Hemp advocates boast of the plant's environmental virtues, arguing that using hemp saves trees, that its pesticide demands are minimal, and that it helps hold soil, thus preventing erosion. However, should hemp be intensively cultivated as a cash crop, cautioned Smith, it would require nitrogen fertilizer, much the same as other cash crops. Another potential drawback is that hemp plants contain silica, which can damage paper-making machines.
Even though hemp may not be a problem-free crop, it's attractive enough for Scholts to explore. He is planning a second forum on hemp. "If this can be a valuable crop for American agriculture, we have to keep moving forward and investigating it," he says.
A study released in the April 1995 American Journal of Public Health, authored by NIEHS statistician Beth Gladen and epidemiologist Walter Rogan, shows evidence for a correlation between DDE levels in milk and shortened lactation in 229 Mexican women. DDE is the most stable derivative of the pesticide DDT, which is banned in most of the world, including the United States and Mexico. Since DDT is persistent in the environment, women, and consequently breast-fed babies, may still be subject to its effects.
Infants face serious health risks if their mothers suffer from shortened or failed lactation. Decreased lactation has been associated with increased infant mortality, especially in developing countries, but effects are also detectable in the developed world, including the United States. In developing areas where water may be contaminated, feeding babies with powdered milk instead of breastfeeding may leave them vulnerable to diarrheal diseases and other waterborne pathogens, leading to infant mortality. Even where the water is clean and does not present a hazard, many researchers believe that bottle-fed babies may be missing out on important health benefits of breastfeeding, including stronger immune function.
In a 1987 study, the North Carolina Breast Milk and Formula Project, researchers found that while DDE in milk did not show any direct effect on infants' health, those children whose mothers' milk carried high levels of DDE were breast fed for markedly shorter times than those with lower levels. To replicate the study, the authors chose an agricultural region of Mexico where DDE levels were likely to be high due to previous pesticide use--the town of Tlahualilo, in the northern state of Durango. The researchers used local medical personnel to administer questionnaires about the demographic and socioeconomic status of the mother, her medical and reproductive history, the pregnancy, the delivery, attitudes toward breast feeding, and the baby's feeding pattern since birth. The study followed 229 women every 2 months from childbirth until weaning or until the infant was 18 months old. Samples of breast milk were collected just after birth and when the infants were 6 months old.
The fat content of the milk samples was analyzed for p,p´-DDE, the most common isomer of DDE. The correlation between p,p´-DDE and shortened lactation was confirmed, even when factors such as previous lactation and other reasons for weaning were considered. Among reasons for weaning were illness of the mother, use of oral or injected contraceptives, and the mother's perception that the child was old enough to be weaned. The most common reason for weaning was insufficient milk.
Since infant illnesses showed no correlation with DDE levels, the researchers reasoned that DDE was somehow tampering with the process of lactation itself. According to the report, "The most plausible explanation of a relationship between DDE and duration of lactation is estrogenicity." While the p,p´-DDE isomer which the researchers measured is nonestrogenic and shows no effect on lactation in rats, it is assumed to occur in proportion to another isomer, o,p-DDE, a weak but persistent estrogen in several animal models. DDE and other pesticide residues may mimic the activity of natural estrogens, which occur at high levels during pregnancy but fall just after birth to allow lactation. Medically administered estrogens can also decrease or halt lactation.
Gladen emphasizes that although this epidemiological study established a correlation between DDE and shortened lactation, the mechanism for DDE's action is still unknown. Gladen would like to see a good animal model for lactation developed that could be used to test chemicals including DDE. Says Gladen, "We need to understand the underlying mechanism more before we can block DDE's action."
An extensive analysis of 11 studies of radon-exposed miners shows that radon exposure in U.S. homes may account for as many as 14,400 lung cancer deaths a year, about 10% of American lung cancer victims. Residential exposure to radon, an odorless, invisible gas emitted by the decay of uranium in the earth's crust which can accumulate in enclosed areas, has been identified as the second leading cause of lung cancer in the United States. Because radon occurs naturally, exposure cannot be totally eliminated. However, some 2,000- 4,000 U.S. lung cancer deaths per year may be prevented if all homes with radon levels exceeding the EPA's action level were repaired, the analysis estimates. A summary of the analysis was published in the June 7 issue of the Journal of the National Cancer Institute.
The joint analysis, which pooled original data on some 65,000 miners in China, Canada, Europe, and the United States, confirms the findings of an earlier National Academy of Sciences BEIR (Biological Effects of Ionizing Radon) IV report based on 360 lung cancer deaths among 2,700 miners. The BEIR IV report helped form the basis for the EPA's recommendations that every U.S. home be tested for radon and that homes with levels above 4 picocuries per liter (pCi/l) be repaired. The EPA estimates that some 6 million homes will have radon levels at or above the 4 pCi/l action level.
"This study confirms that radon is a serious public health problem, as EPA, the Surgeon General, [the Department of] Health and Human Services, and many others have been saying," says David Rowson, director of the EPA's radon division. "It adds robustness to the data we have supporting our recommendations. We now have data on nearly 70,000 humans who have been exposed to radon."
The joint analysis, based on more than eight times the data of the earlier study, represents the most comprehensive look at radon exposure risk to date. The study's authors used a measurement called a "working-level month," or WLM, to compare exposures to underground radon of miners to residential exposures. A working level as defined by the authors is any combination of radon progeny in 1 liter of air that results in the emission of 130,000 MeV (megaelectron volts) of energy from alpha particles. The study found that in an average home, yearly exposure results in approximately 0.2 WLM, or an approximate lifetime cumulative exposure of 10-20 WLM. People who reside their entire lives in a home at the EPA action level of 4 pCi/l accumulate 40-80 WLM. The study also found that from 5-10% of American homes have radon levels that would produce a lifetime exposure equal to 40-50 WLMs.
The study concludes that in miners, about 40% of all lung cancer deaths may be due to radon exposure, accounting for 70% of lung cancer deaths among never-smokers and 39% of lung cancer deaths among smokers. Using a 1993 figure of 149,000 U.S. deaths from lung cancer, the authors estimate that 4,700 never-smokers and 9,700 smokers each year may die of lung cancer attributable to residential radon exposure.
Among the findings of the analysis were that the relative risk relationship for cumulative radon exposure was consistently linear in miners, suggesting that exposures at lower levels, such as in homes, would carry some risk, that the excess relative risk for never-smokers was threefold the trend for smokers, and that for equal total exposure, exposures of long duration and low levels were more harmful than exposures of short duration and high levels.
The study also evaluated in greater detail the effects of other factors on risk, and found, for example, that the relative risk from exposure diminished over time. "Even 25 years after exposure, the risk continued to decline," says NCI health statistician Jay Lubin, the lead author of the report. The analysis also suggests that children are not at any greater relative risk merely because they were exposed as children. And among miners, Lubin notes, "the suggestion is that the person who [is exposed] at low rates for long periods of time may be at a slightly higher risk, although what that means in terms of residential exposure is still unclear."
Estimates of miners' risks are consistent with the seven epidemiological case-control studies that seek to directly measure exposure in homes, says Lubin. But Lubin notes two difficulties with residential studies. Extrapolating from mines to homes, using a lifetime exposure at 4 pCi/l, gives a relative risk of 1.2-1.4, compared to a relative risk of 15-20 for smoking. "The risk is very small, which makes it difficult to detect," Lubin said. Second, measurements in one or two rooms of a home do not precisely characterize lifetime exposure, since people move and exposure outside the home is unknown. "The effect of the imprecision is to totally muddy the water. It becomes much more difficult to establish risk in that kind of setting," says Lubin.
In the face of ambiguous results of case-control studies and confounding factors, the authors urge caution in interpreting the results of the analysis, "because concomitant exposures of miners to agents such as arsenic or diesel exhaust may modify the radon effect and, when considered together with other differences between homes and mines, might reduce the generalizability of findings in miners." Still, the authors insist, the findings do suggest that for certain homes with high levels of radon, "there is no question that remedial action should be taken; as a result, lung cancer risk would be lowered."
Editor's Note: An article by Warner et al. in an upcoming issue of EHP discusses the effect of residential mobility on radon exposure (vol. 103, no. 12).
EHPnet
Scientists around the world are piecing together the puzzle of what causes cancer and how it may be treated. OncoLink (URL: http://cancer.med.upenn.edu/), an award-winning World Wide Web site created at the University of Pennsylvania, offers researchers, clinicians, and patients the latest clues to the puzzle. The goal of Oncolink, the first comprehensive multimedia cancer information resource ever placed on the Internet, is to communicate cancer information worldwide.
OncoLink's homepage contains hyperlinks to information on the latest cancer news, cancer-related meetings, peer-reviewed journal articles on cancer, site statistics, and a search function. OncoLink also has a menu hyperlinked to a variety of submenus. Items on the main Oncolink menu include disease-oriented menus, psychosocial support and personal experiences, cancer causes, screening, and prevention; clinical trials, global resources for cancer information, book reviews, and more.The disease-oriented menu features general cancer groupings for hyperlinks to more specific information on particular types of cancer. For example, under the heading "Adult Cancers," there is a heading for leukemia. Under the leukemia heading there are specific hyperlinks for acute lymphocytic leukemia and acute myeloid leukemia. These links then lead to patient information, transplant information, treatment information, diagnosis, and current research information. Users may also search a particular topic or field of interest.
The bulk of research information is found under the cancer causes, screening, and prevention menu and the global resources menu. From the cancer causes menu, users may link to an environmental factors and causes submenu. This submenu offers hyperlinks to information on topics such as lung cancer and radon, DDT and breast cancer, electromagnetic fields and cancer, and risk assessment. The global resources menu offers links including hospitals, universities, institutes, associations, and government sites related to cancer including the National Cancer Institute's international Cancer Information Center and the EPA.
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