Every Rainy Season the Whole House Falls Ill at Once. It Took Three Years to Learn It Was Not the Weather

Ploy's family lives in a three-storey townhouse in Bang Na, Bangkok. There are four of them: Ploy, her husband, their ten-year-old son, and Ploy's 66-year-old mother, who sleeps in the ground floor bedroom.

Every year since they moved in, the same thing happened once July arrived. Their son began coughing at night. Grandmother developed a wheeze in the mornings. Ploy sneezed and blocked up badly enough to take antihistamines almost daily. Her husband developed an itchy rash around his neck. Then in November, as the air dried out, every symptom gradually faded on its own.

Everyone in the house reached the same conclusion: it was the change in weather. It sounded reasonable, and nobody questioned it, until the third year, when Grandmother had to go to the emergency room after struggling to breathe on a night of heavy rain.

Ploy started reading seriously, then bought an inexpensive temperature and humidity meter for each bedroom. What she saw in the first week made her sit down. The ground floor bedroom held 82 percent relative humidity all day, while her son's third floor room sat at 68 percent.

And the dampest room was, exactly, the room belonging to the person with the worst symptoms.

Why the Rainy Season Turns an Ordinary Mattress Into a Problem

Most people assume rainy season allergies come from cooler air or airborne pollen. The mechanism that actually operates inside a house is quite different, and it comes down to a number you can measure.

1. Humidity is the single variable that governs dust mite numbers

Dust mites do not drink. They absorb moisture directly from the air through their skin. That makes relative humidity the factor determining whether they survive and multiply. Below 50 percent they dehydrate and die. At 75 to 85 percent, the normal range in a Thai home during the rains, their population can multiply several times over within weeks.

2. Mould begins growing at the same humidity level

Above roughly 70 percent relative humidity, mould spores naturally present in the air begin to germinate on any surface containing organic material, which includes cotton fibres in a mattress, sofa foam and carpet pile. Grandmother's ground floor mattress was therefore accumulating dust mites and mould at the same time.

3. The ground floor is always damper than upstairs

Moisture rises through the concrete slab from the ground, and cooler air, which holds less moisture, sinks. Ground floor bedrooms in Thai homes are therefore consistently more humid. This is why, within one house, the person sleeping downstairs usually has the worst symptoms, and that person is often an elderly relative who finds stairs difficult, the most vulnerable member of the household.

4. We close the house up more during the rains

Our own behaviour compounds the problem. When it rains we shut every window, dry laundry indoors because there is no sun, and use the air conditioning less because it already feels cool. All three trap humidity indoors with no route out. A single load of laundry dried indoors releases up to two litres of water into the air.

Why It Is the Weather Is Such a Dangerous Explanation

It makes us stop looking for a cause

This is the real problem with that explanation. Once we believe it is the weather, we accept it as natural and uncontrollable, and turn to managing symptoms with antihistamines. Ploy's family spent three full years in that mode, when the actual driver was humidity and what it was feeding inside their mattresses, something both measurable and fixable.

Three years of accumulated cost

Ploy totalled it retrospectively at about 74,000 baht: antihistamines and inhalers for four people across three rainy seasons, doctor visits, one emergency room attendance, days off work for her and her husband, and eleven days of school missed by their son.

A risk that grows every year

More concerning than the money: repeated, prolonged allergen exposure year after year raises the risk of a child developing asthma, and progressively worsens symptoms in an older adult with existing respiratory disease. Each year that passes without addressing the cause is not simply one more season endured, it is accumulated risk.

And it will certainly return every year

Unlike a one-off problem, this one has a fairly predictable start date at the beginning of the rains. The advantage of that is you can plan ahead, if you know it is coming.

What the Family Tried, and Why It Was Not Enough

• Buying a dehumidifier. The most correctly targeted tool of all, and genuinely useful for preventing things getting worse, but it only controls air humidity. It does not remove three years of accumulated mites and mould spores already inside the mattress.
• Airing mattresses in the sun on dry days. During the rains there are barely any days with strong enough sun for long enough, and even then heat kills only live mites; the carcasses and droppings, the actual allergens, remain.
• Hot-washing bed linen more often. Necessary and worth doing, but it only addresses the removable layer. The mattress itself, the real reservoir, is never touched.
• Running the air conditioning all day to dehumidify. It does reduce humidity, but running it constantly without ventilation recirculates the same particles, and the electricity cost makes it unsustainable.
• Taking antihistamines daily all season. It manages symptoms without reducing allergen load at all, and three to four months of continuous medication every year is not a good long-term plan for anyone.
• Vacuuming the mattress with a household machine. Bag-type vacuums discharge fine particles through the exhaust. In an already damp, closed room, that is direct distribution of mould spores throughout the space.

The Plan That Worked: Remove What Has Accumulated, Then Control the Humidity

Ploy booked the full dust mite removal and disinfection service from World Health Disinfection for the whole house in early June, before the rains fully arrived. It covered all four mattresses, every pillow, the living room sofa, rugs and all curtains, with particular attention to Grandmother's ground floor bedroom.

The core equipment is the SIRENA SYSTEM dust mite vacuum from Canada, with a 1200-watt Italian cyclonic motor, using water filtration together with a HEPA filter capturing particles down to 0.02 microns.

The point that matters especially where mould is concerned: mould spores are extremely small, and an ordinary bag vacuum tends to discharge them back through its exhaust, effectively sowing spores throughout a damp room that is ready to grow them. Water filtration captures everything in water, 100 percent, and the technician empties the dirty water outside the house.

This is followed by a disinfection step using CHEMGENE HLD4H, which eliminates viruses, bacteria and fungi within one minute at 99.85 percent efficacy, with protection lasting up to 14 days, as tested by a department of microbiology and immunology. That step addresses rainy season mould directly.

The SIRENA is certified by the Asthma Society of Canada, with TUV Rheinland certification and Go Green and Eco Friendly accreditation.

Afterwards, Ploy kept running the dehumidifier downstairs to hold humidity below 60 percent, a level at which dust mites struggle to multiply. Doing both together is what actually works: remove what has accumulated, then control the environment so it does not come back.

10 Reasons to Remove Dust Mites Before Every Rainy Season

1. You start the season from a low baseline. The fewer mites remaining when humidity rises, the slower their population grows all season.
2. Mould spores captured in water, 100 percent. Nothing is released back into a damp room ready to grow them, which is the single most common mistake made with ordinary vacuums.
3. A disinfection step follows, addressing fungi directly at 99.85 percent efficacy with 14 days of continued protection.
4. Filtration to 0.02 microns. Covers mite droppings and mould spores, the two principal allergens of the rainy season.
5. It targets the damp ground floor room specifically, which is usually the bedroom of the most vulnerable person in the house.
6. Carpets and curtains treated without dismantling, which almost nobody does during the rains because nothing dries.
7. It removes rainy season musty odour without air fresheners, which typically worsen symptoms for allergy sufferers.
8. It reduces reliance on antihistamines through the season, a better long-term goal for both children and older adults.
9. No harsh chemicals in the extraction stage. Safe for children, elderly relatives and asthma patients in the home.
10. You can plan ahead because you know when the problem arrives. Book in May or June each year, before humidity peaks.

See the full service details and pricing here

Before and After: This Rainy Season Compared With the Previous Three

Note: results depend on the house, humidity levels, bedding age, and continued humidity control after the service.

In the Customer's Own Words

For three years we said it was the weather and went back to buying antihistamines. What I regret most is that one easy explanation stopped us looking for an answer for three whole years. A humidity meter costing a few hundred baht changed everything, because it gave us a number. We now book the dust mite service every early June, the same way we service the car at set intervals.

Ploy, townhouse owner, Bang Na, Bangkok

Frequently Asked Questions About Rainy Season Allergies

What time of year should it be done?

May to June is best, before humidity peaks, so the season starts from the lowest possible baseline. If you have missed that window, doing it mid-season still helps considerably.

What humidity level is safe?

Generally, keep indoor relative humidity between 45 and 55 percent. Below 50 percent dust mites struggle to multiply, and below 60 percent most moulds will not grow. An inexpensive humidity meter is enough to monitor it.

Do I also need to buy a dehumidifier?

Not always, but strongly recommended for ground floor rooms or any room regularly measuring above 70 percent. Dust mite removal resets to zero; humidity control stops it coming back quickly. The two complement each other.

Will the mattress get wet, when nothing dries in the rains?

Water-filtration dust mite extraction is a dry process. The mattress does not get wet and is usable immediately, a significant advantage in a season when wet washing carries a real risk of not drying completely.

Which room should be done first on a limited budget?

Start with the room measuring the highest humidity, usually on the ground floor, and the room of whoever has the worst symptoms, typically an elderly relative or a young child. The team can assess and advise on priority.

Is anything included?

Customers using the full dust mite removal service receive complimentary WELLGIENIC disinfectant wet wipes and CHEMGENE HLD4H disinfectant spray, particularly useful in the rains for wiping surfaces starting to show damp marks.

If Your Household Falls Ill in the Same Month Every Year, That Is Not a Coincidence

Symptoms that arrive on schedule every rainy season have an identifiable cause and can be planned for in advance. It is not something you have to endure for one more season.

See our dust mite removal and disinfection service and pricing here

Free consultation: 065-556-6294
LINE @whd268

World Health Disinfection Co., Ltd., 88 268 Kanlapaphruek Road, Bang Khae, Bangkok 10160

Related Services and Articles

• Dust mite removal, mattress, sofa and carpet cleaning with full disinfection
• Disinfection spraying for residential homes
• Read all our hygiene and disinfection technology articles

External References

• World Health Organization, Asthma Fact Sheet
• Department of Disease Control, Ministry of Public Health Thailand

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