The warning applies specifically to Hardy Pond. It is not a citywide advisory covering every pond or waterbody in Waltham.

Activities residents should avoid

The city says people should not swim, boat, jet ski, swallow the water, fish or consume fish from Hardy Pond. Residents should also keep dogs and other animals away from the water.

Anyone who comes into contact with the pond water should rinse promptly with clean tap or bottled water. That advice also applies to pets. The U.S. Environmental Protection Agency recommends thoroughly rinsing exposed animals immediately and watching for symptoms that may require veterinary care.

Why cyanobacteria can be harmful

Cyanobacteria, sometimes called blue-green algae, occur naturally in freshwater. Under certain conditions, they can multiply rapidly and form a harmful algal bloom. Some cyanobacteria produce toxins, and people should avoid contact during an advisory rather than trying to judge the risk by appearance alone.

Massachusetts health guidance says exposure can happen through swallowing contaminated water, skin contact or inhaling water spray. Possible effects include gastrointestinal symptoms, skin or eye irritation and asthma-like symptoms from inhalation. Children and pets face heightened risk because they may be more likely to get contaminated water in their mouths, and pets can ingest material from their fur after leaving the water.

What to watch for after exposure

People who develop symptoms after contact should contact a health professional. The state advises contacting a doctor if someone swallows the water or develops adverse health effects.

For pets, the EPA lists vomiting, diarrhea, weakness, staggering, drooling, breathing difficulty and seizures among symptoms that require prompt veterinary attention. Rinse an exposed animal immediately and contact a veterinarian if symptoms appear.

What remains unknown

The public notices reviewed for this article do not disclose toxin concentrations, cyanobacteria cell counts or a date for lifting the advisory. Hardy Pond remained on the state’s active advisory list on July 24, so residents should not assume the warning has ended.

Residents should check new updates from the Waltham Board of Health and the Massachusetts Department of Public Health before returning to the pond or resuming recreational activities there.

Sources

• City of Waltham public-health advisory: Hardy Pond
• Massachusetts DPH harmful algae guidance
• U.S. EPA pet-safety guidance