Every year, Delhi’s air quality hits a new record for severe pollution, putting millions of residents at familiar and emerging health risks. Data on particulate matter, dust, and vehicular emissions are shared and dominate conversations. But new research emphasises the need to look beyond the usual chemical pollutants. Airborne microbial contamination, especially antibiotic-resistant bacteria, is a rising concern that has so far flown under the radar.

A new study, published in Scientific Reports, analysed indoor and outdoor air samples from various urban settings in Delhi and found significantly higher levels of Staphylococcus, a group of bacteria commonly associated with multidrug resistance.

“When we talk about air pollution, we typically measure parameters such as carbon dioxide, sulphur dioxide, nitrogen dioxide, PM2.5, lead, and others,” says Madhuri Singh, a research scientist in the School of Environmental Sciences, Jawaharlal Nehru University, and corresponding author of the study. “However, microbial load should also be considered as an additional parameter, as it is equally important from a public health perspective.”

The link between antimicrobial resistance and air pollution is not new; the science is well established, and the evidence is clear. However, this particular study is an important addition to the existing research as it uses data from Delhi, one of the cities that battles severe air pollution year after year, to show this link, says Sarah Hyder Iqbal, researcher and co-founder of Superheroes Against Superbugs, a community for awareness and action against antimicrobial resistance.

Understanding antimicrobial resistance

Antimicrobial resistance (AMR) is a growing global public health concern in which bacteria, viruses, fungi, and parasites evolve to resist antimicrobial medicines. While modern medicine is heavily dependent on antibiotics, misuse and overuse of antimicrobials have led to a rise in drug-resistant pathogens.

It is well known that people are exposed to various bacteria in the air, but there is a stark difference when those bacteria harbour antibiotic resistance genes, which can have serious health implications. For one, it could put the benefits of modern medicine at risk, the World Health Organization (WHO) warns. AMR can make infections such as urinary tract infections, which are usually easier to treat with antibiotics, increasingly challenging when bacteria develop resistance.

AMR is a particularly significant problem in India, where the misuse and indiscriminate use of antibiotics is more common than in other countries, Singh explains. “Additionally, the improper disposal of antibiotics, along with effluent from hospitals and pharmaceutical industries, is contributing to a residual load of antibiotics in our water bodies and soil,” she adds.

Moreover, these low amount of antibiotics find their way back into the air through re-aerosolisation, driven by wind, flushing, and other environmental processes. While these concentrations are too low to kill bacteria, they are enough to place bacteria under some pressure, giving them a window of opportunity to develop resistance mechanisms and acquire mutations that allow them to survive antibiotic exposure. Once a bacterium develops resistance, it can transfer that resistance to other, previously susceptible bacteria, spreading antimicrobial resistance across ecosystems, Singh elaborates.

Link between AMR and air pollution

In a 2023 study in The Lancet Planetary Health, researchers collected data from 116 countries between 2000 and 2018 to provide the first global estimates of the correlation between particulate matter (specifically PM2.5) and AMR. The findings showed that the level of PM2.5 has a stronger statistical association with aggregate antibiotic resistance than does antibiotic use or access to drinking-water services.

The study showed that a 1% increase in PM2.5 pollution was associated with an increase in antibiotic resistance of 0.5-1.9%, depending on the pathogen.

While research continues to strengthen the link, the relationship between AMR and air pollution remains understudied and underdiscussed.

To better understand the risk of airborne microbes in air pollution, Singh and team, in their recent study, looked at the presence and diversity of staphylococci in bioaerosols, airborne particles carrying biological material, from various urban settings in Delhi, including Vasant Vihar Urban Slum, Munirka Market Complex, Munirka Apartment, and the sewage treatment plant at Jawaharlal Nehru University.

The researchers found a staphylococcal load in both indoor and outdoor air, measuring about 16,000 CFU/m³.

While there is no uniform international standard for safe levels of airborne staphylococci, microbial concentrations above about 1,000 CFU/m³ are generally considered high and indicative of contamination. Studies also validate 1000 CFU/m³ as an acceptable threshold for microorganism concentrations in indoor air, though this value is not specific to staphylococcal load.

Still, the numbers paint a troubling picture. The findings showed that 80% of hospital-acquired staphylococci isolates were resistant to penicillin, and nearly half could not be treated with methicillin, a commonly used antibiotic. Moreover, about three-quarters of methicillin-resistant staphylococci were resistant to not just one but multiple antibiotics.

While staphylococci are not harmful to everyone, they are opportunistic, Singh says. “This means that they are not harmful to healthy people. But they become a concern for people with compromised immunity, such as the elderly, young children, people suffering from a disease, or those using medical devices. People with skin infections or open skin wounds are also more vulnerable,” she further explains.

However, Shraddha Karve, Assistant Professor (Research) at the Koita Centre for Digital Health, Ashoka University, points out that there is a need to rethink what a ‘healthy individual’ means today. “Someone exposed to chronic air pollution, with declining lung capacity and a constantly burdened immune system, may not fit the classical definition of healthy, even if they appear fine on the surface,” she says.

Indoor settings and seasonal changes

The study particularly examined how microbial load varied across seasons and indoors and outdoors. The researchers found that microbial load was highest during winter at the Munirka Market Complex, a crowded place.

“This could be because during winter, there is high pollution, and the particulate matter, PM2.5 and PM10, is also very high. Moreover, particulate matter works as a vehicle for the adhesion of these bacteria; it is a kind of carrier of these bacteria,” says Singh. During this season, there is also a low flow of air and reduced ventilation, which helps the bacteria remain suspended in the environment, she adds.

The staphylococcal load indoors remained relatively high throughout the monsoon season. The researchers note that this is likely due to favourable conditions for microbial growth, such as optimal indoor humidity and temperature. The lowest staphylococcal load occurred in summer, when conditions such as high temperatures, low relative humidity, and intense sunlight could have deterred the microbes.

The study casts doubt on the widely held assumption that indoor environments provide meaningful protection against airborne infectious agents. While indoor settings may reduce exposure to particulate matter, they do not constitute an effective barrier against bacterial transmission. “Just being inside may not bring the desired reduction in the incidence of infection,” Karve says.

Iqbal also points out that the seasonal and spatial differences further bring inequality to the forefront. People with more money can buy tools to protect themselves, such as purifiers or masks. They can afford to miss a day of work without it greatly impacting their livelihood. “But there is a huge section of people who cannot afford to do these things, and they tend to be outdoors and often in crowded spaces. They will be more vulnerable but have less access to resources,” she says.

Interestingly, Kamini Walia, microbiologist and senior scientist at the Indian Council of Medical Research (ICMR), says that the presence of airborne staphylococci does not automatically imply a direct human health risk.

Various studies have documented that microorganisms are present in both indoor and outdoor environments. However, “there is no clear evidence directly linking these observations to quantifiable human health risks,” she says.

Walia further elaborates that the study isolated 836 presumptive staphylococcal samples, of which only two were identified as Staphylococcus aureus, a highly virulent human pathogen.

For these two isolates, the observed antibiotic resistance patterns were not supported by corresponding antimicrobial resistance genes. Because of this lack of concordance, the findings do not provide conclusive evidence of true antimicrobial resistance, she says.

Most of the remaining isolates were coagulase-negative staphylococci, bacteria commonly found on human skin. “These organisms may exhibit pathogenic potential only under certain conditions,” Walia adds.

AMR monitoring and surveillance

Recent data has highlighted India’s pressing need to address AMR. The latest report by the Indian Council of Medical Research (ICMR) showed that methicillin-resistant Staphylococcus aureus (MRSA) rates have steadily increased over eight years of surveillance, from 33% in 2017 to nearly 53% in 2024. Although the anti-MRSA antibiotics, such as vancomycin, worked well in controlled environments.

AMR is a symptom of a really broken health and social system, Iqbal says. “AMR is not a disease. It is not a health condition per se. It exposes the cracks across our health and social systems and the political and economic factors that shape how antibiotics are prescribed, used, and sold; the behaviours around antibiotic use, infection prevention, and the lack of access to affordable, quality, and timely healthcare,” she explains.

Singh emphasises the need to regulate the availability of over-the-counter antibiotics and to enforce stricter controls on antibiotic use. “In areas with a high load of antibiotics and bacteria in the air, people should be advised to wear masks and personal protective equipment, and to minimise the time spent in such environments,” Singh suggests.

Karve also points out that the real burden of AMR in India is concentrated in ICU wards, and that is where surveillance systems are currently focused. But meaningful surveillance needs to go further, she adds. “It must connect what is found in the air to what is being seen in hospitals, specifically, the respiratory infections that are hardest to treat, most prolonged, and carry the highest morbidity and mortality,” she explains.

There are research consortium initiatives such as the Alliance for Pathogen Surveillance India (APSI), and the ICMR has also tried to create an AMR surveillance infrastructure and network. “But none of them has been really effective in real-time monitoring. And a lot of this monitoring is mostly in hospitals, so the focus for AMR has been in hospital settings, not really in the environment,” Iqbal says.

India has acknowledged the “silent pandemic” in its new National Action Plan on Antimicrobial Resistance (2025-29), which prioritises the One Health framework, a unifying approach that recognises the interconnectedness between human health, animal health, and our shared environment. But Dr. Abdul Ghafur, senior consultant in Infectious Diseases, Apollo Hospital, Chennai, wrote that while the new plan “provides the scientific and strategic foundation India needs. But its success will depend entirely on how effectively national and State systems work together.”

The new study shows that the link between AMR and air pollution is complex but cannot take a back seat in discussions, policies, and solutions. “In Delhi, we are already seeing how breathing troubles cascade into a chain of other health problems, and at the centre of it all is a weakened immune system. When your body is denied clean air consistently, internally the body doesn’t function the way it should,” Karve says.

Read more: Pollution concentrations soar higher above ground during haze

Banner image: Commuters amid smog in New Delhi. (AP Photo/Altaf Qadri)

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Kundan Pandey Editor

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