PM1 poses greater health risks than PM2.5 but remains unmonitored in India.
Studies link it to cardiovascular disease, hypertension (also in children), cancer and respiratory illness.
Without standards or technology upgrades, India is missing a critical opportunity to address a major public health threat.
PM1, particulate matter smaller than one micron, is a lethal but least understood air pollutant in India’s toxic airscape. Globally, scientists have warned that this ultra-fine fraction may be causing damage far more severe than PM2.5 and PM10, yet remains entirely outside India’s regulatory framework.
The country has no standards or verified monitoring technology deployed at scale for PM1, even as experts say that it is silently driving cardiovascular disease, hypertension, respiratory illness and premature deaths in polluted cities.
“PM1 is not monitored as part of India’s national air quality monitoring system. India’s continuous ambient air quality monitoring stations (CAAQMS) currently measure PM10 and PM2.5, but PM1 is not included in the CPCB or state-level regulatory monitoring frameworks. Some short-term monitoring is done as part of research studies, but not public reporting,” said Shambhavi Shukla, programme manager, Clean Air and Sustainable Mobility Programme, Centre for Science and Environment.
What makes the ultrafine PM1 especially hazardous is its ability to move past the body’s natural defences in a way larger particles cannot. PM10 is largely filtered out by nasal hair and mucous, while PM2.5 travels deep into the lungs and is already linked to millions of global deaths. PM1, however, is small enough to penetrate the alveoli, cross into the bloodstream and, in some cases, infiltrate through the skin. A growing pool of international evidence is demonstrating just how potent this microscopic pollutant is.
A 2020 Scientific Reports study in Harbin, China, found that PM1 particles carry hazardous chemicals and heavy metals, including lead, cadmium, chromium and nickel, all of which are strongly associated with heart disease and cancer. This toxic load moves through the bloodstream, heightening long-term risks even at low-level exposure.
Immediate impacts are equally stark: A 2017 study published in The Lancet Planetary Health examining emergency room visits across 26 Chinese cities found that even a modest rise of 10 microgrammes per cubic metre (10 μg / m3) in PM1 levels triggered a significant surge in hospital visits, with the effects lasting up to two days after pollution spikes.
Another major multi-city study in 2020 reported that a10 μg / m3 increase raised the risk of heart disease by 0.29 per cent, stronger effect than PM2.5 or PM10.
Children face the gravest risks. Research published in the Hypertension by the American Heart Association in 2020, involving nearly 10,000 children, reported that every 10 μg / m3 rise in PM1 increased the risk of high blood pressure by an alarming 61 per cent, with the youngest and overweight children most affected.
PM1 forms nearly 50 per cent of PM2.5 concentration. Yet, neither the World Health Organization nor India’s Central Pollution Control Board has set standards for PM1, and the pollutant does not appear in any national regulatory framework.
This absence of measurement is creating a major data gap. Studies show PM1 originates largely from high-temperature combustion: Modern vehicular engines, industrial stacks and recondensed organic and metal vapours. In Delhi, it exhibits a tendency to bind to coarser dust that is already a major component of PM2.5.
Further, ultra-fine particles also accelerate haze through atmospheric new particle formation, amplifying pollution within dense urban bubbles, according to studies done in China.
A March 2025 study tried to quantify the PM1 data gap. During humid winter mornings in Delhi, its concentrations are routinely underreported by as much as 20 per cent, which is equivalent to 50 μg / m3, according to the report published in the journal Nature.
This is due to a phenomenon called 'hygroscopic growth', which is the swelling of particles in moist air. This underestimation is most pronounced during rush hours, when emissions and humidity peak simultaneously.
Because human activity makes up the bulk of PM1 sources, densely populated urban areas, especially those with busy roadways or industrial facilities, are especially prone to PM1 pollution as well as other types of particle pollution.
Part of the reason PM1 remains outside India’s monitoring system is technological. According to Abhishek Chakraborty, assistant professor in the environmental engineering department at IIT Bombay, “Research on PM1 is still very limited, and no regulatory bodies have set a limit yet. Although it often constitutes > 50 per cent of PM2.5 mass, it has both health and climatic impacts. The lack of reliable and approved real-time monitoring instruments, the absence of standards and limited evidence and expertise on PM1 sources, composition, and toxicity are likely delaying the PM1 measurements. Infrastructure-wise, you need to upgrade / modify the existing samplers / instruments.”
The health implications of this oversight are significant. “Long term health impacts will depend on the exposure levels, duration, and composition of PM1. Currently, we have limited research on this topic. However, similar to PM2.5, PM1 impacts cardiovascular health negatively, degrades sleep quality, can cause cancer, asthma and is possibly more harmful due to its smaller size and deeper penetration into our respiratory system,” Chakraborty said.
What makes PM1 more harmful than PM2.5 is not only its size but its chemical behaviour. Its extremely small diameter allows it to travel deeper into the lungs and directly enter the bloodstream, circulating to vital organs including the heart, liver and brain. This deep tissue access increases the risk of systemic inflammation, oxidative stress and cellular damage. Oxidative stress occurs when the body accumulates too many unstable molecules called free radicals and lacks enough antioxidants to neutralise them. These excess free radicals begin attacking healthy cells, undermining tissue function and accelerating disease processes.
PM1 also carries more toxic metals and reactive chemicals per unit mass than PM2.5 due to its larger surface-area-to-volume ratio. Epidemiological studies increasingly link PM1 exposure to higher risks of heart attacks, stroke, hypertension, respiratory infections and adverse birth outcomes. And because PM1 remains suspended in air for longer periods and infiltrates indoor spaces more easily, overall exposure is considerably higher.
Despite this, India remains focused on PM10 and PM2.5, pollutants whose dangers are well-established but which no longer represent the full scope of risk. Bharat Stage VI vehicular norms include limits on particle numbers, an indirect nod to ultra-fine emissions, but fall far short of comprehensive PM1 regulation. Without standards or routine measurement, PM1’s contribution to disease remains underestimated and unaddressed.
Chakraborty believes this gap can be closed quickly if policymakers prioritise it. “Technology for PM1 measurements already exists; with some upgrades to the existing infrastructure, it should be able to measure PM1 within a couple of years. The primary challenge is to gather more evidence on the impact of PM1 on health and air quality, and to establish a regulatory standard. PM1 measurement will become mainstream once a regulatory standard is in place. India could be the first country to frame and implement the same, if we move quickly on this.”