Delhi homes contain fungal levels 12 times above WHO limit

As Delhi braces for winter and the likely return of late-fall smog, a study has found that indoor air in city homes contains fungal spores at levels 12 times higher than the World Health Organization (WHO) safety limit, causing skin allergies, respiratory problems, and even anxiety among residents.

The study, conducted by Delhi University's Satyawati College, Jamia Millia Islamia, and the US's South Dakota State University, also revealed that bacterial levels indoors were recorded to be ten times higher than WHO safety limits. Long-term exposure to such high concentrations of fungi and bacteria makes indoor air in many parts of Delhi almost as harmful as the smog outside, noted the study, published in Frontiers in Public Health in 2025.

Most fungal particles were smaller than 2.5 microns small enough to penetrate deep into the lungs and cause lasting respiratory damage. The researchers observed a clear seasonal pattern: fungal levels rose steadily from winter, peaking at around 6,050 colony-forming units (CFU) per cubic metre in fall, between September and November, just before Delhi’s smog sets in.

Bacterial levels increased from winter to summer, peaking in August before declining in fall About 33% of residents reported frequent headaches, 23% complained of burning or irritated eyes, 22% experienced persistent coughing and breathlessness, and 18% suffered from sneezing and allergic rhinitis, also known as hay fever. Around 15% reported skin irritation and itching, the researchers noted.

“Children and young adults emerged as the most vulnerable groups. About 28% of children under 12 and 25% of young adults aged 18 to 30 reported breathing difficulties, coughing, or allergy-related symptoms,” the study added. Developing lungs, higher activity levels, and longer indoor exposure made children particularly susceptible. Women also reported higher rates of eye and skin irritation, accounting for nearly 60% of all dermatological and eye complaints, likely due to spending longer periods indoors.

Titled “Microbial Indoor Air Quality Assessment and Health Correlations in Densely Populated Urban Areas of Delhi, India”, the study found that fungal and bacterial concentrations inside homes in densely populated areas of the national capital were several times higher than WHO safety standards, based on a year-long survey of households in North Delhi.

Researchers collected air samples from 336 houses across the slum clusters of Ashok Vihar and Azadpur, representing low-income, overcrowded neighborhoods with poor ventilation and visible dampness. The survey covered all four seasons to track how microbial levels changed throughout the year. Fungal concentrations ranged from 1,330 to 6,050 CFU per cubic metre, compared with the WHO-recommended limit of 500 CFU per cubic metre up to twelve times higher than safe levels. Bacterial concentrations varied from 730 to 5,300 CFU per cubic metre, exceeding the WHO reference level of 500 CFU per cubic metre by over ten times.

The researchers linked the seasonal surge to humidity and temperature, which create ideal conditions for fungal growth indoors. The most common airborne microbes identified were Aspergillus, Penicillium, Cladosporium, Staphylococcus, Streptococcus, and Micrococcus. Among these, Aspergillus and Cladosporium were dominant fungi known to trigger asthma, allergies, and chronic lung infections. While microbial counts were highest during the late monsoon and fall, health complaints were most frequent in winter and fall, suggesting that prolonged indoor exposure in poorly ventilated conditions worsened outcomes.

A health questionnaire administered to 509 residents, using a structured survey approved by the Indian Council of Medical Research (ICMR), revealed widespread health issues linked to poor indoor air. Researchers attributed Delhi’s poor indoor air quality to overcrowding, inadequate ventilation, high humidity, and infiltration of outdoor smog. Many households had visible mold and damp walls, with minimal sunlight exposure. Meteorological and outdoor pollution factors also played a role.

The study noted that fungal and bacterial counts increased sharply when humidity exceeded 60–70%. The authors warned that the health risks extend beyond allergies and asthma. Species such as Aspergillus and Penicillium produce mycotoxins, chemical compounds that can cause headaches, fatigue, and immune suppression after prolonged exposure. Chronic inhalation of fine fungal particles may also lead to hypersensitivity pneumonitis, a severe inflammatory lung condition often mistaken for asthma or bronchitis.

According to the authors, indoor air quality remains a neglected area of environmental health policy in India. They called for the establishment of national indoor air quality standards and urged improvements in ventilation systems, especially in low-income, high-density housing areas. The study concluded that indoor pollution, particularly from fungal bioaerosols, represents an “unseen but potent health risk” amid Delhi’s ongoing air quality crisis.