
How Are Rainfall and Flooding Related to Allergies and Asthma?
Key Takeaways Rain and flooding can alter the environment around us. Increased humidity creates favorable conditions for house dust mites and mold, while flooding can lead to persistent indoor dampness and increased exposure to respiratory irritants. Under certain thunderstorm conditions, pollen may rupture into much smaller allergen-containing particles that can penetrate deeply into the lower airways. Respiratory infections are also important triggers of asthma exacerbations. For people with asthma, particularly those sensitized to house dust mites, mold, or pollen, maintaining good asthma control, continuing prescribed controller medications, and having an individualized Asthma Action Plan are important during periods of heavy rainfall and flooding, as well as during the post-flood recovery period. 1. High Humidity Creates a Favorable Environment for House Dust Mites House dust mites are among the most common indoor allergens, particularly in warm and humid climates such as Thailand. They are commonly found in mattresses, pillows, sofas, carpets, soft toys, and upholstered furniture. House dust mites depend on environmental moisture for survival. Therefore, indoor humidity is an important factor influencing their survival and population. People who are sensitized to house dust mites and have allergic rhinitis or allergic asthma should pay particular attention to reducing indoor humidity and minimizing exposure to house dust mite allergens, especially during the rainy season. 2. Flooding, Dampness, and Mold The effects of flooding on allergic diseases may persist even after floodwater has receded. Moisture can remain trapped in walls, ceilings, flooring, carpets, mattresses, and furniture, creating a persistently damp indoor environment that promotes mold growth. Medical evidence has demonstrated an association between residential dampness and mold and asthma, as well as respiratory symptoms such as cough and wheezing, particularly in susceptible individuals and those with pre-existing asthma. 3. Flooding Also Brings “Respiratory Irritants” Floodwater can introduce mud, dust, sewage contaminants, microorganisms, and other irritants into homes and buildings. After the water recedes, removing water-damaged materials and cleaning affected areas may disturb dust, mold particles, and other airborne contaminants. These exposures can irritate the nasal and bronchial mucosa and may aggravate symptoms in people with sensitive airways or asthma. The effects of flooding are not limited to the respiratory system. Prolonged exposure of the skin to water and moisture, as well as extensive wet work during post-flood cleaning, may impair the skin barrier. Exposure to detergents, disinfectants, and other cleaning agents may further increase the risk of irritant contact dermatitis. 4. “Broken Pollen” and Thunderstorm Asthma Rain does not always reduce airborne allergen exposure. Thunderstorm asthma events have been reported in several regions worldwide, with the large Melbourne epidemic in Australia. Under thunderstorm conditions, especially during periods of high airborne pollen concentrations, certain pollen grains may absorb water and rupture through osmotic processes, releasing much smaller allergen-containing particles. These smaller particles can penetrate more deeply into the lower airways, potentially triggering sudden and severe asthma exacerbations in susceptible individuals. 5. Barometric Pressure, Weather Changes, and Asthma Before and during heavy rainfall or thunderstorms, several meteorological factors may change simultaneously, including barometric pressure, temperature, humidity, wind patterns, and airborne allergen concentrations. These meteorological changes can influence the distribution of aeroallergens, air pollution, temperature, humidity, and airway responses. Together, these factors may affect individuals with airway hyperresponsiveness and asthma. 6. Respiratory Infections Rain and flooding can alter the environment around us. Increased humidity creates favorable conditions for house dust mites and mold, while flooding can lead to persistent indoor dampness and increased exposure to respiratory irritants. Under certain thunderstorm conditions, pollen may rupture into much smaller allergen-containing particles that can penetrate deeply into the lower airways. Respiratory infections are also important triggers of asthma exacerbations. For people with asthma, particularly those sensitized to house dust mites, mold, or pollen, maintaining good asthma control, continuing prescribed controller medications, and having an individualized Asthma Action Plan are important during periods of heavy rainfall and flooding, as well as during the post-flood recovery period. References 1. Burbank AJ, et al. Climate Change and the Future of Allergies and Asthma. Curr Allergy Asthma Rep. 2025. 2. Gu W, et al. Association of humidity and precipitation with asthma: a systematic review and meta-analysis. Front Allergy. 2024;5:1483430. 3. Luedders J, Poole JA, Rorie AC. Extreme Weather Events and Asthma. Immunol Allergy Clin North Am. 2024 Feb;44(1):35-44. 4. Thien F, et al. Thunderstorm Asthma: Current Perspectives and Emerging Trends. J Allergy Clin Immunol Pract. 2025. 5. Le Souëf PN, Adachi Y, Anastasiou E, et al. Global change, climate change, and asthma in children: Direct and indirect effects— A WAO Pediatric Asthma Committee Report. World Allergy Organ J. 2024 Nov 8;17(11):100988. 6. Urbani F, Cometa M, Martelli C. et al. Update on virus-induced asthma exacerbations. Expert Rev Clin Immunol. 2023 Jul-Dec;19(10):1259-1272. บทความโดย Anchalee Senavonge, M.D. Allergy and Immunology, BNH Asthma and Allergy Center










