In last week’s Outbreak Thursday post, we discussed the types of infectious disease threats that those affected by Hurricane Harvey might face as a result of the record-setting rain and flooding. In addition to the spread of communicable diseases in the close quarters of evacuation shelters, waterborne infectious diseases via flood waters (which may not be as great of a threat as one might think), and vectorborne diseases as receding water leaves behind ideal breeding grounds for mosquitoes, hurricane victims and responders may also face health threats due to mold exposure during recovery and cleanup operations. In this edition of Outbreak Thursday, we evaluate the potential risk to human health posed by molds and other fungi in the wake of hurricane-related flooding.

Molds are present throughout the indoor and outdoor environment, and require warm, moist environments to flourish. As the Houston flood waters recede, wet structures and materials (eg, carpet and walls) coupled with warm temperatures (90°F on Tuesday this week) will provide a perfect environment for the growth of molds and other fungi. An estimated 30,000 to 40,000 homes were destroyed by Hurricane Harvey, and many, many more likely sustained at least some water damage. These homes will need to be repaired, gutted, or demolished, and as demolition activities take place, spores from mold and other fungi will become airborne, potentially posing significant health risks. Additionally, some estimates suggest that upward of a million cars—the wet upholstery of which also provides an excellent environment for mold to grow—were destroyed by the hurricane. Removal and destruction of these cars could also place individuals at risk due to mold contamination.

The growth of mold on houses and other structures following a hurricane has been well documented. After Hurricane Katrina, the CDC found visible mold growth in 46% of surveyed houses and heavy mold growth in 17% of houses—“correlating with the extent of wind and water damage”—which extrapolates to 194,000 homes and 70,000 homes with visible mold growth and heavy mold growth, respectively, across the affected area. This study likely underestimates the severity of mold growth, however,, as the areas hit hardest by the hurricane were inaccessible.

But what are the likely health effects from all that mold? One study of US Coast Guard responders during Hurricanes Katrina and Rita found significant associations between self-reported mold exposure and sinus infection, respiratory symptoms, difficulty concentrating, depression, diarrhea, digestive health effects, and joint pain. Another study after Hurricane Katrina found a more than threefold increase in the prevalence of both upper and lower respiratory symptoms in children and adolescents compared to before the hurricane, with significant correlations with self-reported exposure to mold, dust, or smoke/fumes. Additionally, several case reports describe acute fungal sinusitis and central nervous system blastomycosis in individuals who worked in hurricane-affected areas.

Though the reports above are worrisome, there is also evidence that mold-related health conditions may not be as prevalent after hurricanes as might be expected based on the high prevalence and severity of mold growth that often occurs. One meta-analysis did not find an association between adverse health outcomes and exposure to mold following Hurricanes Katrina and Rita. The authors identified several potential reasons why this association was not stronger. First, they note that mold does not typically cause severe disease in healthy people, and healthy adults are more likely to perform demolition or cleanup activities. Second, it is possible that people affected by hurricanes experience lower levels of exposure than anticipated. The time that individuals spend conducting cleanup activities may be relatively limited. The CDC study cited above found that individuals who participated in heavy and light cleaning after Katrina did so for an average of only 13 and 15 hours, respectively, and 39% did not stay overnight in their homes during the cleanup. Additionally, many people are forced to find alternate housing, temporary or permanent, during the recovery as a result of flood or wind damage or electricity or water outages.

Despite not finding a significant increase in mold-associated illnesses following hurricanes, it is possible that the above meta-analysis does not fully account for the true incidence of disease that occurs. As the authors note, people experiencing mild symptoms may not seek medical care, which means their illness will not be diagnosed or reported. Additionally, many people struggle with access to health care following a hurricane, and those that seek care outside of the affected area may not be captured by local surveillance systems. Furthermore, local health departments often do not routinely collect data on mold-related conditions, so it may be difficult to fully characterize the impact of hurricanes and associated flooding on the prevalence and incidence of these conditions without targeted data collection and analysis efforts. Several studies cited above also note the difficulty of assessing the level of environmental contamination due to mold—particularly for airborne samples—during the recovery process after a hurricane, which further complicates efforts to determine correlation between mold exposure and specific health effects. Many studies rely on self-reported exposure to mold in the absence of the ability to collect reliable quantitative exposure data. Finally, longer-term health effects from mold and other fungal exposure may take months or years to detect, further complicating efforts to link the onset of health conditions to specific exposures.

The CDC has published a guide (in English and en Español) on mold remediation after flood-related events, which provides detailed recommendations regarding mold removal techniques, personal protective equipment (eg, goggles, respirators, gloves), and debris disposal. Hard, nonporous surfaces can be cleaned relatively easily, but it is much more difficult to remove mold from saturated wood, drywall, fabrics, and other similar materials. While PPE such as gloves, eye protection, and respirators are widely available to the public, proper use of these products is critical to mitigating the risk of exposure to mold and other harmful substances. One study conducted in New Orleans after Hurricanes Katrina and Rita found that only 20% of surveyed individuals used N95 respirators properly, with 17% of individuals donning the respirator upside down.

Local health officials in Houston are actively engaging with the public as they attempt to rebuild their homes and lives to promote safety measures that can protect them against unforeseen risks and help reduce the long-term health effects of Hurricane Harvey. Interestingly, more than 250,000 people were relocated to Houston after Hurricane Katrina, with 25,000 to 40,000 remaining there permanently, only to be affected 12 years later (almost to the day) by Hurricane Harvey. So far, Houston-area hospitals and health officials have reported an uptick in the “number of patients with injuries suffered during clean-up efforts, respiratory problems including pneumonia and, most of all, skin infections contracted by those who spent time in the water.” While the increase in patients being seen by hospitals is actually lower than what officials were expecting, the noted rise still highlights that while the storm itself has subsided and the waters are receding, health risks from Hurricane Harvey will continue as recovery and cleanup operations, including demolition, ramp up in the Houston area.

Whether we will see an increase in mold-associated illnesses following Hurricanes Harvey and Irma remains to be seen. Well-designed, prospective studies are needed to better characterize the risk of these types of illnesses following hurricanes.


Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly in near-real time to allow others to learn from these experiences in order to improve global outbreak response capabilities.