Winter is coming, so is smog: Study finds air inside Delhi homes as harmful as outdoors

New Delhi, Oct 6 (PTI) As Delhi braces for winters and likely another spell of the late fall smog, a study has found that the air inside city homes are filled with fungal spores 12 times higher than the WHO safety limit, causing skin allergies, respiratory issues as well as anxiety among residents.

The study by Delhi University's Satyawati College, Jamia Millia Islamia and US's South Dakota State University, has also revealed that the bacterial levels were recorded to be 10 fold higher than the World Health Organization (WHO) safety limits.

Long term exposure to high fungal and bacterial concentrations make indoor air in several parts of Delhi almost as harmful as the smog outside, noted the study published in Frontiers in Public Health, 2025.

The study also noted that most fungal particles were smaller than 2.5 microns, small enough to penetrate deep into the lungs and cause lasting respiratory damage.

It observed a clear seasonal pattern. Fungal levels rose steadily from winter and reached their highest concentration at around 6,050 CFU per cubic metre in fall between September and November -- the period just before Delhi’s smog sets in.

Bacterial levels increased from winter to summer, peaking in August, before declining in fall, it said.

About 33 per cent of residents reported frequent headaches, 23 per cent complained of burning or irritated eyes, 22 per cent experienced persistent coughing and breathlessness, and 18 per cent suffered from sneezing and allergic rhinitis, also known as hay fever. Around 15 per cent reported skin irritation and itching, the researchers noted.

“Children and young adults emerged as the most vulnerable groups. The study found that about 28 per cent of children under 12 years and 25 per cent of young adults aged 18 to 30 reported breathing difficulties, coughing, or allergy-related symptoms,” they added.

The authors noted that 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 per cent of all dermatological and eye complaints, likely because they spent longer periods indoors, they added.

The study, titled “Microbial Indoor Air Quality Assessment and Health Correlations in Densely Populated Urban Areas of Delhi, India”, found that fungal and bacterial concentrations inside homes in densely populated parts of the national capital were several times higher than safe standards set by the WHO, 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 neighbourhoods with poor ventilation and visible dampness.

The survey covered all four seasons to track how microbial levels changed throughout the year.

The findings revealed that fungal concentrations ranged between 1,330 and 6,050 colony-forming units (CFU) per cubic metre, while WHO recommends a limit of 500 CFU per cubic metre, meaning that homes recorded up to twelve times higher fungal levels than what is considered safe.

Bacterial concentrations varied from 730 to 5,300 CFU per cubic metre, compared with the WHO’s reference level of 500 CFU per cubic metre, showing that bacterial counts were also more than 10 times higher than global safety limits.

The researchers linked this seasonal surge to humidity and temperature, which create ideal conditions for fungal growth and persistence indoors.

The most common airborne microbes identified were Aspergillus, Penicillium, Cladosporium, Staphylococcus, Streptococcus, and Micrococcus. Among them, Aspergillus and Cladosporium were dominant fungi known to trigger asthma, allergies, and chronic lung infections, they said.

While microbial counts were highest during the late monsoon and fall, health complaints among residents were most frequent in winter and fall, suggesting that prolonged indoor exposure in poorly ventilated conditions worsened health outcomes.

A health questionnaire was administered to 509 residents of the sampled homes, using a structured survey approved by the Indian Council of Medical Research (ICMR). The results showed widespread health issues linked to poor indoor air.

The researchers attributed Delhi’s poor indoor air to a combination of overcrowding, inadequate ventilation, high humidity, and infiltration of outdoor smog. Many households showed visible mold growth and damp walls, while sunlight exposure was minimal.

Meteorological and outdoor pollution factors also played a crucial role.

The study noted that as humidity levels rose above 60 to 70 per cent, fungal and bacterial counts increased sharply.

The researchers warned that the health risks associated with indoor fungi 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.

The study also cautioned that chronic inhalation of fine fungal particles that are present in the city's homes can 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 and high-density housing areas.

The study concluded that indoor pollution, particularly from fungal bioaerosols, represents an “unseen but potent health risk” in Delhi’s ongoing air quality crisis. PTI SGV VIT NB NB