DDPHE Blog: Avoiding Algae Blooms and Mosquito Bites this Summer

DDPHE Blog: Avoiding Algae Blooms and Mosquito Bites this Summer

Published on July 17, 2025

Summer in Denver means more time outside, enjoying Denver's beautiful parks or - enjoying a warm evening in your backyard. However, warmer weather also presents some health risks that Denverites should be aware of: harmful algae blooms in ponds and lakes and mosquito-borne illnesses, such as West Nile virus.

Here's what to watch for and how to protect yourself and your pets.

Blue-Green Algae: What's That Stuff in the Water?

Algae are a natural part of our lake ecosystems, but under the right conditions, certain types-called blue-green algae or cyanobacteria-can produce toxins that are dangerous to both people and pets. These blooms can look like:

• Pea soup or spilled paint
• Green, blue-green, or reddish-brown scum
• Mats or streaks on the water surface

DDPHE regularly tests the water in Denver's lakes, but blooms can appear quickly. If you see water that looks suspicious, stay out and keep your pets out too. Exposure can cause rashes, stomach issues, breathing trouble, or more serious illness. If contact does happen, rinse off immediately and monitor for symptoms. You can help prevent blooms in Denver's waterways by using phosphorus-free lawn products and keeping fertilizers off streets and sidewalks. Learn how to stay algae aware by listening to DDPHE's Environmental What If podcast.

West Nile Virus: What's the Buzz About Mosquitoes?

From May through the first hard freeze-usually in October-mosquitoes are active in Denver. Some can carry West Nile virus, the most common mosquito-borne disease in the continental U.S. DDPHE monitors mosquito activity weekly from mid-June through mid-September and uses larvicide to reduce mosquito populations in Denver. Avoid pesky bites and unwanted illness by protecting yourself with the 4Ds:

• Defend: Use insect repellent like DEET to keep mosquitoes away.
• Dress: Wear long sleeves, pants and closed-toe shoes, especially during peak mosquito hours.
• Drain: Empty standing water in buckets, gutters, flowerpots and toys around your home each week to prevent breeding sites.
• Dusk & Dawn: Mosquitoes are most active at these times-limit outdoor time when possible.

Listen to the mosquito episode of the Environmental What If podcast to hear DDPHE experts discuss how to prevent the development of adult mosquitoes, how to safely treat mosquito habitats, and best practices to repel them.

While everyone is at risk of contracting West Nile virus, individuals over 50 years old or with weakened immune systems are at higher risk of developing serious illness. Most cases are mild, but if you experience serious symptoms, especially if you're over 50 or have a weakened immune system, contact a healthcare provider.

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