While floods are known to cause immediate dangers such as drowning, injuries, and hypothermia, experts warn that long after waters recede, communities remain at risk, particularly from respiratory health problems linked to water damage and mould growth.
Heavy rainfall and rising sea levels, both exacerbated by climate change, are driving more frequent and severe flooding events globally.
According to a report from Harvard Health, such incidents not only destroy infrastructure but also leave behind invisible threats that can affect respiratory health for weeks, months, or even longer.
“Flooding may bring water contaminated with toxic chemicals, heavy metals, pesticides, biotoxins, sewage, and water-borne pathogens into buildings,” the report states.
“Afterward, some toxic contaminants remain in dried sediments left behind. When disturbed through everyday actions like walking and cleaning, this turns into microscopic airborne dust.”
“This means anything in that dried flood sediment — the toxic chemicals, the metals, the biotoxins — is now in the air you breathe into your lungs, potentially affecting your respiratory health,” the report adds.
Water intrusion, mould growth pose serious risks
Buildings do not need to be completely submerged to pose respiratory hazards. Researchers found that many homes damaged during Hurricane Ida experienced water intrusion through windows, roofs, and air vents, even over 100 miles away from the storm’s epicentre.
Another major post-flood hazard is mould. This fungal growth thrives in moist environments, particularly following water damage. As mould spreads by releasing tiny spores into the air, exposure to high concentrations indoors can lead to health complications such as asthma, sinusitis, and allergic rhinitis.
“No indoor space is entirely free from mould spores, but exposure to high concentrations is linked with respiratory complications,” the report says.
Following Hurricane Katrina in 2005, researchers recorded average outdoor mould spore levels in flooded areas of New Orleans at nearly double those in unaffected areas. Indoor concentrations were even higher.
Studies also show that children are particularly susceptible. In one study conducted in 30 Canadian communities, children in homes with dampness or mould reported significantly higher rates of asthma, bronchitis, coughing, and eye irritation.
Preventive measures can make a difference
Harvard Health researchers studying Hurricane Ida found that two key factors strongly influenced whether indoor mould developed: the age of a building’s roof and the number of preventive actions taken after flooding.
“The impact on respiratory health also varied with flood water height, days per week spent at home, and how many precautionary measures were taken,” the report explains.
Based on their findings, experts suggest a series of actions both before and after flooding to reduce risks:
Before flood season:
- Repair roof damage, clean gutters, and seal vulnerable areas like skylights, vent pipes, and chimneys.
- Clear drains and ensure septic tanks are emptied.
- Seal cracks in external walls and around windows.
- Install a sump pump in the basement and backflow valves on sewer lines.
After flooding:
- Avoid direct contact with flood water, which may contain raw sewage and pose electrical risks.
- Delay re-entry into flood-affected homes until they are dried and declared safe.
- Inspect for dampness and signs of water intrusion or mould.
- Drain all water and dispose of sediment promptly.
- Remove and, if possible, dry water-damaged porous materials in sunlight.
- Maximise ventilation by opening windows and using exhaust fans.
- Use dehumidifiers in enclosed or damp spaces.
- Upgrade HVAC air filters to MERV 13 or use portable air cleaners with HEPA filters.