San Francisco -- Just because your tap water stinks and tastes bad doesn't mean it is dangerous to drink, the San Francisco Public Utilities Commission says after dozens of people complained about a musty aroma and odd flavor.

There is blue-green algae in the water, but not enough to cause any health problems, according to Andrew DeGrace, the water quality division director for the commission.

"We're saying that people should not be concerned," DeGrace said. "The levels we are talking about are so small that it is not a health-related issue."

The water district had to increase the amount of water it was taking from the Calaveras Reservoir, on the Alameda-Santa Clara county line, in an attempt to lower the water level so workers can begin a multi-million dollar dam replacement project, DeGrace said.

The Calaveras water has a higher level of Aphanizomenon, a common form of blue-green algae. Blue-green algae can become toxic in high concentrations, but DeGrace said the amount that is in the water that utility district customers in San Francisco and the Peninsula are getting does nothing more than give the water a musty odor and taste.

The public utilities commission usually uses about 11 1/2 gallons of Hetch Hetchy water to each gallon from Calaveras. The new blend is roughly 3-to-1, he said.

Still, DeGrace said, only 9 algae cells per milliliter are in the water, a level that is acceptable in drinking water. The sudden change nevertheless prompted 40 customers to lodge complaints over the past few days, he said.

"A few days ago I noticed my tap water really stinks and tastes bad in an unusual way," e-mailed one customer who identified herself only as Janis. "I asked a few neighbors, and they notice the smell in their taps, too."

The Mission District woman said she has been suffering from debilitating allergy problems for the past week that she thinks may have been caused by the algae-laden water.

The water district will continue pumping Calaveras water until the beginning of next week, but DeGrace said the odor and taste problem will probably subside before then.

"It's still good water," he said. "We have exceptionally high qualities of water, so for us this is an elevated level (of algae), but for most water districts it's a low level."