After Milton made landfall as a Category 3 hurricane in western Florida last week, some residents who evacuated the area are now returning to discover water damage in their homes. And they may soon find something else: mold.

Mold can begin to grow as soon as 24 to 48 hours after moisture exposure. And Florida’s subtropical climate, which remains hot and humid in October, makes it a perfect breeding ground. Even worse, mold will continue to grow until the source of moisture is eliminated.

As climate change ushers in heavier rainfall and more intense storms, mold is likely to become a more common problem. Warmer temperatures add moisture to the atmosphere, which can produce more intense precipitation events. “Mold is absolutely associated with climate change,” says Mary Johnson, a principal research scientist of environmental health at Harvard University. “Those extreme weather patterns, including hurricanes, can allow water to come into a home or any type of indoor space. When it’s damp, the likelihood of mold growing indoors increases.”

Mold growth has previously been documented after major storms that caused flooding and water damage in residential neighborhoods. A few months after Hurricane Katrina hit New Orleans in 2005, researchers with the US Centers for Disease Control and Prevention inspected 112 homes affected by flooding and found that nearly half had visible mold growth. And Johnson’s research has linked higher temperatures and increased precipitation to more mold spores in outdoor air.

A type of fungi, mold is a natural part of the environment and has been on Earth for millions of years. Outside, it plays an important role by decomposing organic material, such as dead trees and leaves. But indoors, mold can be a health risk, especially to children, people with allergies and asthma, and those with weakened immune systems.

Mold spores—reproductive cells that act like seeds—travel through the air both outdoors and indoors. When people breathe in these spores, they can cause respiratory symptoms, itchy eyes, infections, or skin rashes. Inhaling them can also trigger asthma attacks that can be serious. Not everyone is affected by mold exposure, though.

“Our bodies, for the most part, deal with mold,” says Ronald Saff, an internal medicine doctor, allergist, and immunologist in Tallahassee, Florida. “People are exposed to molds at fairly high concentrations if they go for a walk in the park or in wooded areas.”

For most healthy people, short-term mold exposure isn’t usually a problem. But for individuals with asthma and other chronic lung diseases, or those who are immunocompromised—including transplant and chemotherapy patients—it’s not a good idea to go into a moldy home. Mold can worsen underlying respiratory diseases and cause fungal infections in the lungs of people with weakened immune systems. With the highest percentage of residents age 65 and older in the nation, many Floridians may be at risk of getting sick.

Over a long period of time, even healthy people who are exposed to a wet, moldy, indoor environment may be at risk of developing allergies or asthma, Saff says. A comprehensive study conducted by the National Academy of Medicine and released in 2004 found there was sufficient evidence to link indoor exposure to mold with upper respiratory tract symptoms, such as coughing and wheezing, in otherwise healthy people. More recent studies have suggested that children exposed to indoor mold are more likely to develop asthma.

Scientists agree that mold can cause respiratory problems for some people, but there’s little evidence that it can induce fatigue, dizziness, headaches, and nausea—a collection of symptoms sometimes referred to as “sick building syndrome” and blamed on indoor mold. And contrary to popular belief, Stachybotrys chartarum, commonly known as black mold, is likely no more dangerous than other types of mold.

Ebrahim Ahmadisharaf, an assistant professor of civil and environmental engineering at Florida State University, has been studying residential flooding and mold growth after hurricanes to learn more about its potential impacts on respiratory health. He and a team of hydraulic engineers and public health scientists surveyed 60 water-damaged homes in New Orleans and Baton Rouge, Louisiana, as well as in South Florida, New York City, and Philadelphia after Hurricane Ida in 2021 and Hurricane Ian in 2022.

“It’s not just these extreme events that are becoming more frequent and intensified,” Ahmadisharaf says. “You can also have these chains of small rainfall events happening that can contaminate your indoor air.”

His team is aiming to find the major drivers of mold growth and their correlation with asthma and allergy symptoms. They first gathered information from homeowners on building characteristics, such as roof conditions and ventilation systems, as well as flood levels around their homes and how long flooding lasted. They then visited the properties and collected indoor and outdoor air and dust samples, from which they were able to identify more than 40 species of mold. They also collected data on whether occupants had respiratory symptoms or visited the hospital post flooding.

The team used machine learning models to help parse the data, which showed that the age of a roof, maximum flood depth, use of exhaust fans and their airflow rate, and how often residents opened windows were all important factors for mold growth. Ahmadisharaf says most of the mold issues researchers noticed were in low-income households. The research, which is supported by the National Science Foundation, has not yet been published in a peer-reviewed journal.

For those returning to a water-damaged home, Saff says, there’s usually no need to pay hundreds of dollars for mold testing. Mold is easily identifiable by sight—it looks like fuzzy, slimy, or powdery spots—and by its musty smell.

He recommends sealing up moldy parts of the home with a tarp and creating a separate sleeping area if possible. Ventilate the area with open windows or fans. HVAC systems can spread mold, so they shouldn’t be turned on until mold has been removed.

“We start seeing the threat of mold whenever there’s moisture and excessive water,” says Jae Williams, director of communications for the Florida Department of Health. “So certainly, after a storm like the two that we’ve just had in the last couple of weeks, there’s a lot of storm surge and also some flooding.”

The Federal Emergency Management Agency can provide financial assistance to help low-income homeowners and renters repair or replace mold-damaged homes following hurricanes.

For those tackling cleaning their homes without professional help, the CDC recommends wearing an N95 respirator, gloves, and eye protection. Bleach or dish detergent can be used with water on hard surfaces. Affected areas should be allowed to dry completely. Carpet, upholstered furniture, and other absorbent materials that are moldy may need to be replaced.

“If it’s not dealt with,” Williams says, “it’s just going to continue to become a bigger problem and more pronounced in your home.”