A sheriff in Southern Colorado posted a frightening map on social media this month. It depicted an angry red zone, dotted with flame icons, where wildfires were burning. And a blue zone nearby showed where floods threatened, too.

The graphic wasn’t from the National Weather Service or Federal Emergency Management Agency. It came from a smartphone app.

The mapping and forecasting provided by this app, Watch Duty, are quickly becoming indispensable for emergency authorities, utility crews and residents in parts of the country that, until recently, seemed relatively safe from fires and floods alike.

Watch Duty began culling disaster data from government warnings, weather forecasts, radio scanners and social media five years ago to help residents of a few California counties track wildfires and plan evacuations. Since then, it’s boomed.

In January 2025, after a firestorm swept through Los Angeles, millions of new users joined. Utilities and philanthropic groups have sent millions of dollars to help expand the app’s focus. And this year, it has started to cover wildfires on the East Coast and widespread threats of sudden catastrophic flooding in other regions, too.

The changing climate means existing systems for dealing with disasters haven’t always kept pace with hazards. But Watch Duty is growing more like a tech start-up than the public-service-oriented nonprofit it is.

“People are hungry for information,” said Yusra Kauppila, a regional captain for Watch Duty, one of the dozens of paid staff members and volunteers who gather and vet the data shared with users.

The app has attracted 10 million unique users this year, the organization said, putting it on a pace to surpass the nearly 17 million users it reported in 2025.

Despite recent floods in Texas and fires in Minnesota, 2026 has been a relatively quiet period for disasters in the United States. But there’s been a steady surge in climate-change-fueled wildfires and flooding rains over the past several decades. The most damaging disasters cost the United States more than $100 billion last year, according to Climate Central, a nonprofit group.

Global warming, caused by fossil fuel emissions and the greenhouse effect, is increasing both the atmosphere’s capacity to produce heavy downpours and its ability to rapidly dry out fire-prone landscapes. A developing and potentially record-setting episode of the El Niño climate pattern could intensify some floods over the next year, while diminishing Atlantic hurricane risks.

Elected officials and disaster preparedness groups have raised concern about the government’s ability to respond to such emergencies. President Trump has significantly cut budgets and staff for agencies like the Weather Service and FEMA since he returned to office.

Watch Duty relies heavily on all levels of government to keep its users updated, said John Clarke Mills, its co-founder and chief executive. But it still provides a service those agencies cannot, using technological know-how that the government hasn’t been able to muster as quickly.

“If they’re not going to do it, we’re going to do it,” Mr. Mills said. “If they put us out of business, great.”

Demand keeps rising.

As many as 1,000 people were subscribing per hour to alerts in Florida and Georgia as wildfires spread across the South this spring, giving Watch Duty 127,000 users there within about six months of launching coverage.

The organization began tracking floods in June. A few weeks later, more than a foot of rain fell on parts of Missouri within a few hours, and its page dedicated to the incident quickly garnered 37,000 views, the nonprofit said. The day had started with only about 300 subscribers in the area, Ms. Kauppila said.

Watch Duty is free, but users can pay $25 or $100 annually for upgraded memberships that allow subscriptions to alerts in unlimited numbers of U.S. counties, or extra layers of mapping data on variables such as public land ownership. In 2025, more than 111,000 people paid for the $25 tier, twice as many as a year earlier.

Some companies have found the app valuable, too. Watch Duty said it signed $1 million in recurring contracts with utilities, telecommunications companies and railroads last year, and it expects to more than double that this year.

Pacific Gas and Electric, the utility serving 16 million Californians, is among its heaviest users. PG&E admitted responsibility for the deaths of more than 80 people in the 2018 Camp fire in Butte County, Calif., which utility equipment ignited.

More than 4,000 of PG&E’s 29,000 employees use an internal version of Watch Duty that maps out power infrastructure alongside the app’s fire data, giving them information that helps them protect themselves, communities and the utility’s equipment, said Logan Monroe, the utility’s senior manager of wildfire risk management.

Evaluating how power systems and fires might interact once required hours of cross-referencing internal maps with fire data, Mr. Monroe said. Now, Watch Duty sends fire alerts to his smartphone.

“That’s a three-second investigation,” Mr. Monroe said.

Other apps finding an audience during disasters include MyRadar, popular among tornado watchers, and American Red Cross Emergency, which helps people prepare for and recover from disasters. The National Oceanic and Atmospheric Administration, which includes the Weather Service, has its own radar app and issues wireless alerts for tornadoes and flash floods.

Even with Watch Duty, some communities could struggle to keep up with evolving disaster threats. In Eastern Kentucky, for example, officials are still trying to improve their readiness for unpredictable and deadly floods like those that hit in 2022.

Watch Duty’s new flood coverage could help. But steep topography, limited hydrology data and a lack of access to smartphones and cell service could nonetheless make it difficult to ensure the next flood will come with enough warning, said Rebecca Shelton, director of policy for the Appalachian Citizens’ Law Center, a nonprofit law firm that advocates for climate resilience.

“We really need something that is going to wake people up in the middle of the night, or give them advance notice so they’re watching,” Ms. Shelton said. “We need better info at the community level that’s actionable.”

Mr. Mills acknowledged the challenge, but he said Watch Duty’s rapid growth in areas facing fresh disaster threats shows it’s serving people’s needs.

If a quiet fire or hurricane season means fewer people use Watch Duty, that’s OK with Mr. Mills, he said. “The app isn’t built for attention,” he said. “It’s built for safety.”

As thick smoke from Canada’s wildfires envelopes wide swaths of the country, sending people indoors, there comes a nagging question for many people: If it’s that bad out there, what about the air inside my home?

Wildfire smoke is very bad for you. The toxic air particles from the smoke can cause coughing and wheezing, headaches, stinging eyes, a scratchy throat, sinus irritation, chest pain, rapid heartbeat and fatigue.

These particles are “much, much smaller than the width of your hair,” said Peter DeCarlo, a professor of Environmental Health and Engineering at Johns Hopkins University. “When you breathe them, they can get deep into your lungs.” People with chronic respiratory illnesses, like asthma, are at serious risk, according to the Centers for Disease Control and Prevention, as are children, older adults and pregnant people.

Experts recommend staying inside and closing windows and doors when the air quality drops. However, wildfire smoke can still enter residential buildings, including through natural ventilation, mechanical ventilation and building facades, according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

Here’s what you should know about keeping the air inside safe.

How can I find out what my home’s air quality is?

Unless you have air-quality monitors, you won’t know exactly how healthy (or not) the air is inside your home, but you can keep an eye out on the air outside to inform your decisions.

Monitoring the Air Quality Index, or A.Q.I., will clue you in to what to expect. The air is considered hazardous when the quality index, which measures the density of five pollutants, tops 300. Anything above 150 is considered “unhealthy.” Most weather apps report the A.Q.I., according to Masoud Ghandehari, a professor of urban systems engineering at New York University.

Dr. DeCarlo said that while he loved the idea of everyone’s having an air-quality sensor, the device is an investment that “may give you more understanding of what’s happening, but it’s not going to protect you.”

“If you’re going to invest in something, invest in something that’s going to clean the air,” he said.

Can the outside air enter through my HVAC system?

Generally, it’s not possible for smoky outside air to contaminate the air inside a home through a central air-conditioning system because most of them do not have a dedicated outdoor air intake, said Mike Gallagher, a member of ASHRAE and president emeritus of Western Allied Corporation. But he cautioned that “if there is duct leakage in an attic, for example, then there may be smoke brought in if the attic is ventilated.” And some newer HVAC systems, such as those that prioritize energy efficiency, do have ventilation systems that bring in outside air.

Regardless of which system you have, it’s important to have the right kinds of filters in your HVAC systems, Dr. DeCarlo said, noting that homeowners and renters should look for filters with a high Minimum Efficiency Reporting Values, or MERV, rating.

“The higher the number, the better they are at filtering out,” he said. “Once you get to 10 and higher, you’re going to be able to filter out particles more efficiently in your own air-conditioning system.”

Watch for what Dr. DeCarlo called “pressure drop” in HVAC systems. While a MERV rating of 13 or higher means a filter will take out a majority of concerning particles, older systems might not be able to handle those filters.

“If you think of a filter as a tighter weave of all of these different fibers that are capturing the particles, the tighter that weave gets, the harder it is to get air through it, and older systems may not have the power or the fan strength to push air efficiently through a higher-rated MERV filter,” he said.

What if I have a window unit?

Older homes with leaky enclosures have a higher risk of increased particulate matter concentrations, according to ASHRAE. The group also noted that window and portable air-conditioning units should be avoided or used only in recirculation mode, which reuses the air already within a space.

If you don’t have an HVAC system, you can buy or build an air cleaner, Dr. DeCarlo said. The D.I.Y. versions are called Corsi-Rosenthal boxes and cost about $100 to make. “It’s a box fan, duct tape, and you need four or so of these higher-rated MERV filters to make kind of a cube,” he said.

In a post on LinkedIn, Rich Corsi, the dean of the College of Engineering at the University of California, Davis, who is a co-creator of the design, also noted that air quality may be improved by duct-taping rolls of activated carbon over supply and return vents in your home.

When should I use an air purifier?

Air purifiers should be used during increased air pollution, such as high-smoke events like the one currently blanketing parts of the United States and Canada. How many you need depends on the type of purifier.

Purifiers come in various sizes and strengths, Dr. Ghandehari said, noting that the appliances typically indicate the size of the space that they can clean.

“If you keep the door open to another room, then you have to realize that you’re not only serving the room where it’s located but the next-door room as well,” he said.

Dr. Ghandehari said that people can isolate themselves in the parts of their homes where they have air purifiers and that they should make sure there are no gaps through which smoky air can seep in.

How else can I keep my home safe?

Depending on your location and proximity to the source of the smoke, there’s a chance that you’ll need to wash your curtains, wipe down your blinds or even cover your furniture with a sheet that can be washed later, Dr. Ghandehari said. As examples, he pointed to Michigan and Wisconsin, where the A.Q.I. reached over 400 in some places. If smoke particulate embeds itself into the upholstery, he said, “every time you hit the upholstery, then it kind of raises in the air.”

Places farther away from the source of the current smoke, like New York, don’t necessarily need to be concerned with this because “some of it goes up in the upper atmosphere so the concentration comes down,” he said.

Smoke from the Canadian wildfires is causing hazy orange skies and a red-tinged sun. The reason behind those colors has to do with physics.

The sun emits light in a rainbow of different wavelengths, or colors. Longer wavelengths appear red to our eyes, while shorter wavelengths look blue. Sunlight reaching us on Earth first passes through our atmosphere, which is made of molecules that more readily scatter shorter wavelengths of light in all directions. That’s why our sky usually looks blue during the day.

But during wildfires, an abundance of smoke is also present in our atmosphere. Smoke particles are larger than air molecules and are able to scatter longer wavelengths of light. This causes the sky to tint red.

Dust, water vapor and other types of particles will also scatter longer wavelengths of light, which is why the sky can change color after volcanic eruptions or in places with high amounts of air pollution.

Atmospheric scattering creates other effects in our skies. When the sun is close to the horizon, its light travels through more of the atmosphere, causing more blue light to scatter away. Red and orange light is more likely to pass through, painting our sunrises and sunsets in dramatic hues.

It also comes into play during a total lunar eclipse, when Earth passes between the sun and the moon. Light from the sun spills over the edges of Earth and filters through its atmosphere, illuminating the face of the moon and causing it to flush crimson.

Soaring temperatures are expected to collide with wildfire smoke across parts of the United States this week, a combination that can pose health risks.

As climate change drives up global temperatures past record levels, the frequency of days when it is both hot and polluted has also been increasing.

Several factors are converging to amplify health risks during sweltering days.

Extreme heat can trap polluted air in place, so it does not dissipate. Climate change is making wildfires more frequent and intense, pumping smoke into the atmosphere and contributing to the reversal of a decades-long trend of air quality improvement in some parts of the United States.

In addition, the Trump administration has issued sweeping rollbacks of environmental regulations. It has moved to weaken limits on emissions from power plants and vehicles, which could increase carbon dioxide, sulfur dioxide, particulate matter and other pollutants. It has also issued orders compelling several aging coal plants to stay open past planned retirement dates, extending pollution from those plants. Last year, coal emissions in the U.S. increased after years of decline.

Even as heat can make air pollution worse, the combination of the two environmental hazards can have outsize effects on public health, experts say.

A 2023 analysis of more than 20 million deaths across the world found that hot days and days with bad air quality both resulted in higher-than-normal mortality rates. But periods in which heat and pollution are combined were even more deadly.

The combination can be particularly stressful for children, older people or anyone with respiratory diseases like asthma. Dr. Mary Rice, director of the Center for Climate, Health and the Global Environment at the Harvard T.H. Chan School of Public Health, called the phenomenon a “double whammy” that can result in increased hospital admissions.

“I think of heat as especially bad for the brain, for cardiovascular and mental health admissions,” she said. “Air pollution is very harmful for respiratory and cardiovascular admissions and also stroke. Those together can further enhance cardiovascular and respiratory risk.”

How heat worsens air quality

There are two main types of air pollution that experts track for their effects on human health: ground-level ozone and particulate matter.

Summer days tend to see higher concentrations of ground-level ozone, which forms when sunlight cooks pollutants from vehicles and industrial sources. Ozone irritates the lungs and can cause coughing and shortness of breath.

Particulate matter refers to tiny pieces of solids and liquids in the air, all smaller than a grain of sand or a strand of human hair. When inhaled, the fine particles, known as PM 2.5, can reach the deepest part of the lungs and pass into the bloodstream, causing harm throughout the body, including the heart, lungs and brain.

Extreme heat can also contribute to the conditions that make wildfires more severe. Wildfire smoke contains particulate matter, and can travel for thousands of miles. A 2024 study found that wildfire smoke may have killed as many as 12,000 Californians prematurely in 2018.

“The intensity of that exposure tends to be quite high,” said Dr. Meredith McCormack, director of pulmonary and critical care at Johns Hopkins University and a representative of the American Lung Association.

When buildings burn, the smoke contains metals and chemicals, Dr. McCormack noted. “You have a mixture of natural and man-made sources often during wildfire events at levels that are really extraordinary,” she said.

Regulatory rollbacks

In February, the Trump administration revoked the government’s legal authority to fight climate change, a move that eliminated limits on greenhouse gas emissions from motor vehicles and cleared the way for other regulatory rollbacks.

And last year, Republicans in Congress also eliminated tax breaks that supported the growth of renewable energy, while the Senate blocked California’s plans to implement clean air standards that would have phased out the sale of new gas-powered cars by 2035.

The Trump administration has been promoting the production and use of oil, gas and coal, the burning of which is the main driver of climate change. Mr. Trump has been especially focused on trying to revive the flagging coal industry.

“The continued operation of coal-fired power plants is harmful to the health of those communities,” Dr. Rice said. She added that there was “very clear evidence” that pollutants from coal emissions increased the risk of asthma and could be deadly.