As dark clouds roll over and monsoon downpours begin, Nepalis brace for familiar disruptions: flooded streets, landslides and waterborne illness. Nepal currently endures the peak of its monsoon season, with the Kathmandu Valley recording around 500 mm of rainfall in July alone, a deluge that has submerged streets and drainage systems. But in the old timber-framed homes of Bhaktapur, in the rented flats of Kirtipur and Thamel, something equally perilous is quietly taking root in our walls, on furniture, and deep within our damp structures: indoor mould.

Long dismissed as a seasonal stain or a musty smell to be wiped away before guests arrive, mould slowly poisons the air of our homes and accumulates silently in our bodies across years of chronic exposure, unseen and unmonitored.

Moulds are fungi that release aeroallergens, mycotoxins and volatile organic compounds, which are public health hazards. Outdoor mould gets into homes through openings or by hitchhiking on clothes and pets. These spores float and establish new colonies on damp surfaces. Penicillium, Cladosporium, Aspergillus, Alternaria and Stachybotrys (black mould) are most common, but significant harm comes from their invisible aerospores.

Mould requires only three things: moisture, organic material, and warmth. Nepal’s monsoon provides them all in abundance. A research in Kathmandu found visible mould in bathrooms, kitchens and the basement, with high humidity and poor ventilation linked to respiratory and allergic illness among residents. Another 2016 study of rural mountains found 80 percent of sampled homes fell below the minimum ventilation standard. Mud brick, compressed earth and untreated timber, the building materials of millions of Nepali homes, are hygroscopic. They absorb and retain moisture, making them an ideal substrate for fungal colonisation.

Grain in cloth sacks, herbs and spices in open jars, woollen blankets folded away, wooden furniture, leather shoes and books can all become mould-colonised during the monsoon. When your house smells musty, it’s not an aesthetic quirk of the climate; you are living inside a live bioreactor.

A silent health hazard

A 2021 study published in the Journal of Microbiology revealed that indoor air contains fourfold higher airborne fungi than outdoor air. Penicillium, Cladosporium and Aspergillus were the most common. Their spores act as aeroallergens, triggering responses ranging from sneezing and rhinitis to severe asthmatic attacks. The WHO’s 2009 Indoor Air Quality guidelines and the US Institute of Medicine both establish evidence of indoor mould exposure to upper respiratory symptoms and asthma exacerbations. Particularly alarming, however, are mycotoxins—toxic compounds released by moulds including Aspergillus, Stachybotrys and Penicillium.

Even low-dose chronic exposure induces cognitive impairment, immunosuppression and elevated risks of cancer. According to the 2024 research study in the journal Environmental Health Perspectives, people residing in damp, mould-infested homes reported higher incidences of anxiety and depression. The elderly and infants are vulnerable to mould exposure because of weakened immune systems and increased exposure to mould spores from spending more time indoors. Exposure to mould early in life has also been associated with elevated prevalence of childhood asthma.

In Nepal, where respiratory illness remains a leading cause of under-five-year mortality, the health impacts of indoor mould are likely underdiagnosed. Household pets also introduce mould spores on their fur and may show signs of toxicity for human occupants. Mould spores are ubiquitous in nature and are usually harmless at low levels.

The actual health hazard arises from persistent indoor mould proliferations in damp and unventilated areas, where colonies constantly release concentrated allergens/mycotoxins, resulting in chronic exposure.

Mould can be prevented through the adequate ventilation of interior spaces: opened windows, exhaust fans, increased airflow, speedy leak repairs, dry clothes outdoors, and ample sunlight while trying to maintain indoor humidity levels under 60 percent via use of dehumidifiers where feasible. Mould infested on hard surfaces can be cleaned with diluted white vinegar or bleach, though mould on porous substances such as plaster, timber or mud often needs replacement.

Need for policy reform

Individual practices alone won’t abate mould hazards without policy reform. Nepal’s building codes (NBC) remain ill-equipped to cope with indoor moisture and health: NBC 206:2024 minimally addresses basic lighting and ventilation provision but lacks damp-proofing requirements and exempts vernacular homes; NBC 204:2015 dictates a damp-proof course and drainage specifications, only to frame them as seismic protection rather than health safeguards. The codes ought to outline standards for ventilation and damp-proofing in health-focused housing, and the Right to Housing Act 2075 should include safety from hazardous mould as part of adequate shelter.

The US EPA links mould to 21 percent of national asthma cases; California legally requires landlords to disclose known mould. Japan’s 2003 Building Standards Law mandated 24-hour mechanical ventilation in all new homes as a direct public health response to sick building syndrome. India’s CPWD specifications include dedicated damp-proofing sections for public housing. The UK’s ‘Awaab’s Law’, named after a toddler who died from black mould exposure, requires landlords to investigate and remediate mould within days. Nepal’s municipalities should move towards comparable protections.

Mould proliferates where regulation, awareness and remediation are lacking. To ensure people’s health, indoor air quality can be considered with the same seriousness as structural safety. Until adequate housing standards, public education and legal protections come into effect in Nepal, millions of families will unknowingly be exposed to invisible health risks. Recognising mould as a housing and health hazard is no longer optional; it’s overdue.