SAN FRANCISCO (CBS SF) — If you’re planning on swimming in California waterways over Labor Day weekend, be sure to steer clear of the dozens of waterways compromised by toxic algae blooms.

Thirty-three bodies of water in California have blue-green algae blooms this summer and state health officials are urging recreational water users to avoid contact with affected waters.

Blue-green algae blooms, also called cyanobacterial blooms, have been reported across the state, including the San Francisco Bay Area. Bay Area counties that have reported blooms are Sonoma, Alameda, Contra Costa and Santa Cruz counties, according to the California Water Board’s current list of harmful algal blooms.

Harmful cyanobacteria detected in Lake Shasta; caution urged with contact through Labor Day. https://t.co/c5n0aCizxW pic.twitter.com/I5ylz80gIb — CA Water Boards (@CaWaterBoards) August 25, 2016

On Wednesday, California Department of Public Health director Dr. Karen Smith urged anyone considering swimming, boating, or fishing in reservoirs, lakes and streams in California to avoid contact with waters containing blue-green algae. The warning extends to people’s pets, which are more likely to swallow the water.

On Thursdy, health officials announced harmful cynobacteria had been detected at Lake Shasta and urged caution. Swimming is not prohibited, but adults should ensure that children and pets not swallow lake water containing algae-related material.

Many of the affected waters have posted signs indicating the presence of blue-green algae, according to Dr. Smith.

In the East Bay, where temperatures can reach over 100 degrees Fahrenheit, nine toxic blue-green algae blooms have been reported this summer in the waters at Discovery Bay, Lake Anza, Lake Temescal, Lake Chabot, and Arroyo Del Valle, among other locations.

Drought conditions and climate change are creating hospitable environments for toxin-producing cyanobacteria. These toxic blooms are threatening drinking water supplies and causing wildlife and domestic animal deaths, the California Water Quality Monitoring Council announced last week.

ALSO READ: Toxic Algae Blooms Spurred By Fertilizer Runoff Plague California

Dr. Rosalina Stancheva, the chief scientist at the Primary Algae Laboratory for California, located at California State University – San Marcos said cyanobacterial blooms tend to be present in warmer waters where nutrient levels are high.

“Reducing the nutrient load in freshwater ecosystems is crucial for controlling the cyanobacterial blooms,” Stancheva told CBS San Francisco. “Reducing the use of fertilizers and limiting the agricultural run-off into natural fresh waters is important.”

Since the blooms tend to grow and shrink depending on conditions, not all affected areas have posted signs, so recreational waters users should avoid waters with brown or white foam, scum or mats of blue-green algae, state health officials said. Touching or swallowing the water can cause blue-green algae poisoning that can cause flu-like symptoms as well as eye irritation, skin rashes, mouth ulcers, vomiting, and diarrhea.

Swallowed water containing blue-green algae can negatively impact the human nervous system and cause serious injury to the liver and kidney. If affected water is swallowed, medical treatment should be sought immediately, according to state health officials.

Water purification techniques such as filtering, boiling or adding tablets to the water will not remove the toxins.

State health officials are also urging people not to eat bivalves, such as mussels, from the affected waters. Eating fish should also be avoided or limited. And if fish from those waters are to be consumed, the guts and liver should be removed and the fish fillets should be rinsed in clean drinking water.

Greg Gearheart with the state’s Water Resources Control Board told CBS San Francisco on Thursday that algae blooms and toxic algae blooms have been documented in California for decades, but this summer is the first time the board has released a map of current harmful algae blooms statewide.

Gearheart said that while it’s difficult to pinpoint exactly what causes toxic algae blooms, “there is a linkage to human input of nutrients” and that preventing runoff of fertilizers, which contribute to high levels of nutrients in the water, could help mitigate these blooms.

Unfortunately for recreational water users, once the waterways are impacted by toxic algae blooms, health officials merely urge people to stay away and don’t try to eradicate the blooms. Instead, state health officials, and recreational water users, must simply wait for water conditions to change.

By Hannah Albarazi – Follow her on Twitter: @hannahalbarazi.