State officials have posted a water-quality warning for Mascoma Lake, after tests showed elevated levels of cyanobacteria, also known as blue-green algae, in the lake.

The state advises against swimming in the lake, but does not warn against boating or fishing. Lakes, ponds and coastal waters in New Hampshire are routinely monitored for cyanobacteria, which can be harmful to people and pets.

But it was tip from a concerned citizen in Enfield that triggered the warning for Mascoma Lake, according to Jody Connor of the New Hampshire Department of Environmental Services.

Connor said that until last week, state officials had not issued a cyanobacteria warning for Mascoma Lake in at least five years. He attributed the recent Mascoma Lake warning to heightened vigilance following press coverage of research into a suspected link between cyanobacteria and clusters of ALS, or Lou Gehrig's disease, near Mascoma Lake and several other New England lakes and ponds.

"We've been flooded with calls here," Connor said. "At one point, I had 40 calls that I had to call back."

However, ALS is not the reason why public health officials post warnings about cyanobacteria. At this point, a link between ALS and cyanobacteria is just a hypothetical question, with no hard evidence of a cause-and-effect relationship.

"That is research that hasn't even been completely substantiated yet. It's just beginning," said Sonya Carlson, director of the state's beach monitoring program.

Other health hazards of cyanobacteria are well known.

Cyanobacteria contain toxins that can cause a variety of health problems, including liver damage, neurological effects such as loss of coordination or numbness, muscle pain, nausea, vomiting, diarrhea and skin irritation.

Cyanobacteria were thought to be algae until scientists discovered that they are actually a kind of bacterium. The name blue-green algae comes from their color -- bright green or turquoise -- and resemblance to algae.

Most bodies of water worldwide have some cyanobacteria.

"It's something that is everywhere," said Carlson. "It's only when there's a lot of them that it's a problem."

When cyanobacteria mass at the surface of a lake or pond, it's called a bloom. In cyanobacteria blooms, concentrations of toxins in the water can be high enough to make people sick and have been known to kill pets and livestock.

The kind of cyanobacteria toxin that's most commonly found in New England lakes and ponds is called microcystin, which can affect the liver and nervous system.

To get sick from cyanobacteria, a person has to either drink water or inhale water droplets containing the toxin. People swimming in water where there's a cyanobacteria bloom can accidentally swallow small amounts of water. According to a report from the World Health Organization, there is also evidence that water-skiers or jet-skiers can get sick from inhaling contaminated spray.

Carlson said that there are no known cases of people getting sick from cyanobacteria toxins in New Hampshire. But that doesn't mean it hasn't happened, she said. It's just that the symptoms are so general that it would be hard to connect them directly to swimming or drinking contaminated water, she said.

Vermont state toxicologist Bill Bress said that people might have been sickened by cyanobacteria in Vermont, but that the Department of Health has not documented any cases.

"The symptoms replicate so many other things, they're probably not reported," he said.

Health officials say that cyanobacteria only pose a health risk when they form visible masses on or near the surface of the water.

"If you see green or blue-green scum in the water, stay out of it, and call us," Carlson said.

Both states have systems in place to warn people of cyanobacteria blooms.

In New Hampshire, the Department of Environmental Services relies on volunteers to sample ponds and lakes throughout the state once a month during the summer. Water scientists look at the concentration of cyanobacteria cells in the samples. The state will issue a warning if 50 percent or more of the microscopic organisms in the sample are cyanobacteria, a standard set by the World Health Organization.

Bress said that the Vermont Department of Health does only routine testing for cyanobacteria in Lake Champlain, where dogs have died from ingesting the toxin in lake water.

"The formal procedure that we have is to recommend town health officers to keep an eye out for blooms," he said.

If there's a concern, the state lab tests water samples for microcystin. A warning is posted if the microcystin level exceeds a threshold developed by the WHO.

Bress said that Vermont health officials have not posted any cyanobacteria warnings for ponds or lakes in the Upper Valley in the past several years.

In New Hampshire, warnings are in effect for Mascoma Lake, Goose Pond in Canaan, and two other inland water bodies in the state.

"Our warnings are really focused on primary contact, which is swimming," Carlson said.

The state also warns against drinking the water, but Carlson said that drinking untreated lake or pond water is never a good idea because of all the other harmful things, such as fecal bacteria, that can be present.

The study of a possible link between clusters of ALS and cyanobacteria is being carried out by a group of researchers at Dartmouth-Hitchcock Medical Center, the University of New Hampshire, the New Hampshire Department of Environmental Services and researchers in Wyoming.

The Dartmouth researchers have mapped out several spots near lakes and ponds that have unusually high rates of ALS.

ALS is a degenerative neurological disease with no known cause or cure. Although microcystin can affect the nervous system, the researchers are interested in a different neurotoxin produced by cyanobacteria, called Bmaa (B-Methylamino-L-alanine). A study published in the journal Neurology in 2002 found a potential link between Bmaa and a cluster of ALS in Guam.

"We have not found the actual toxin in the bodies of water," said Elijah Stommel, a neurologist leading the study at DHMC.

In an interview last week, Tracie Caller, a resident physician at DHMC who is involved with the study, said that several potential clusters of ALS have been identified in Upper Valley communities, including Enfield, Windsor, Claremont and New London. She said that the researchers also were looking into potential clusters in Burlington, Vt., and Bangor, Maine.

Caller also provided information from a presentation she gave about the study, which lists potential clusters in Plainfield, Vt., Sommersworth, N.H., and Middleboro, Mass.

The Dartmouth researchers have declined to release a map pinpointing the locations of potential ALS clusters they've found, saying that it was too early to make their preliminary findings public.

Caller stressed that the clusters themselves were suspected, but not statistically proven.

"At this time there is no association between ALS and cyanobacteria, so we have no reason to suggest that people should change their lifestyles," Caller said in an e-mail yesterday.

Carlson said she thinks that as long as people understand that no ALS link has been proven, the recent attention to cyanobacteria has been helpful, in that it has highlighted other known health risks.

She said that "there's no way to predict" how long the current cyanobacteria warnings would be in effect. "It could be a week or a couple days," she said.

But she said that warnings apply to the whole body of water for which they're issued, because wind tends to move the blooms around the shoreline.

She said that people should take reasonable precautions to avoid cyanobacteria blooms.

"Not that it's going to jump out of the lake and kill you," she said. "You're not going to die if you accidentally touch it."

Martin Downs can be reached at mdowns@vnews.com or (603) 727-3210.