How Industrial Emissions Add to City Smog

Stand on a rooftop in an industrial belt like Kanpur, Ludhiana or the outskirts of Ahmedabad on a still winter morning, and you can often see the smog before you can smell it — a brownish haze sitting low over rooftops, thickest near clusters of factory chimneys. It’s tempting to assume that haze is the emission, drifting unchanged from stack to street. It isn’t. What you’re looking at is the end product of a chemical journey that starts the moment hot exhaust gas leaves a chimney and ends, hours or days later, as fine particulate matter suspended over an entire city.

Understanding that journey — not just the headline “industry pollutes” — is what separates a useful explanation from a slogan. This piece follows one industrial emission from combustion to smog, then breaks down which industries contribute the most, and what regulation is actually doing about it.

Step 1: What actually comes out of a factory stack

Industrial combustion — whether it’s a thermal power plant burning coal, a brick kiln, a steel furnace, or a captive diesel generator backing up a factory’s grid supply — releases a mix of gases and particles, not a single pollutant. The main components are:

Sulphur dioxide (SO₂), from burning sulphur-containing fuels like coal and furnace oil
Nitrogen oxides (NOₓ), formed whenever fuel burns at high temperature in the presence of atmospheric nitrogen
Primary particulate matter (PM), soot and ash released directly as solid particles
Volatile organic compounds (VOCs), unburnt or partially burnt hydrocarbons
Carbon monoxide (CO) and, of course, carbon dioxide (CO₂)

None of these, on their own, is “smog.” Smog is what happens after this cocktail meets sunlight, humidity and other pollutants already in the air — which is the part most explanations skip.

Step 2: The chemistry that turns gas into visible haze

This is the step that actually creates the smog you can see.

Secondary particulate formation. SO₂ and NOₓ don’t stay as gases indefinitely. Over hours to days, they react with ammonia (much of it from agricultural and vehicular sources) and atmospheric moisture to form secondary particulate matter — specifically sulphates and nitrates — which are themselves a major component of PM2.5. Crucially, this means an industrial emission doesn’t have to contain particulate matter directly to end up contributing to the particulate haze over a city; the gas transforms into particles after release. Studies on Delhi’s air chemistry consistently identify this secondary particulate formation as a dominant contributor to wintertime PM2.5, often exceeding the contribution of directly emitted (primary) particles during peak pollution episodes.

Photochemical smog. Separately, NOₓ and VOCs react in sunlight to produce ground-level ozone, the main ingredient of the classic “photochemical smog” first documented in cities like Los Angeles and now well-documented in Indian metros too. This reaction needs sunlight, which is why ozone-driven smog tends to peak in the afternoon rather than early morning, unlike the particulate haze that dominates winter mornings.

Why winter makes this worse. The chemistry above happens year-round, but its visible result is far worse in winter for a purely physical reason that has nothing to do with how much industry is emitting: cold, stagnant air near the ground traps pollutants close to street level instead of letting them disperse upward, a phenomenon called temperature inversion. We cover this mechanism — and why it hits north India especially hard — in our companion piece on why winter pollution spikes are worse in north India than the south. Industrial emission volumes don’t necessarily rise in winter; their visible, breathable concentration does, because the atmosphere stops clearing them away.

Step 3: How far does it travel — and does it stay “industrial”?

By the time SO₂ and NOₓ have converted into secondary particulate matter, the resulting PM2.5 can travel tens to hundreds of kilometres from its source, carried by regional wind patterns. This is a big part of why Delhi’s own analyses estimate that only about 20% of the city’s pollution originates within city limits, with the remaining 50–60% arriving from surrounding regions and neighbouring states — a mix that includes industrial clusters well outside municipal boundaries. A factory in one state can measurably worsen the AQI reading of a city hundreds of kilometres away, which is one reason why airshed-level regulation (rather than city-by-city rules) has become a policy priority — something we look at in our breakdown of India’s National Clean Air Programme.

So how much of city smog is actually “industrial”?

This is where honest reporting has to include some uncertainty, because source-apportionment studies — the scientific method used to trace pollution back to its origin — don’t fully agree with each other.

For Delhi specifically, one widely cited 2019-based assessment attributes:

Source category Estimated share of annual PM2.5
Industry (including power plants) ~22%
Transport ~19%
Residential (cooking, heating) ~14%
Agricultural residue burning (annual average) ~7% (rising to ~24% in November)
Other (construction, waste burning, biogenic, etc.) Remainder

But these numbers move a lot depending on the study. Across five major source-apportionment studies examined by researchers, transport-sector estimates ranged from 17.9% to 39.2%, and industrial-sector estimates ranged from just 2.3% to 28.9% for PM2.5 — a spread wide enough that “industry causes X% of smog” should always be read as a study-specific estimate, not a fixed constant (CIEU Aironomics 2025).

What’s consistent across nearly every study, however, is that industry sits among the top two or three source categories in most Indo-Gangetic Plain cities — alongside transport, dust and, seasonally, biomass burning. It is rarely the single largest source, but it is almost never a minor one either.

The industries that matter most

Not all industrial emissions are equal. A few sub-sectors dominate the contribution:

Thermal power plants. Coal-fired power stations remain among the largest single-point sources of SO₂ and NOₓ in most Indian airsheds, particularly clusters located near major cities. Flue-gas desulphurisation (FGD) retrofits — mandated but inconsistently implemented across India’s coal fleet — are the primary lever for reducing this contribution.

Brick kilns. Traditional fixed-chimney brick kilns, common on the outskirts of Indo-Gangetic Plain cities, are inefficient, low-temperature combustion sources that produce disproportionately high particulate and black-carbon emissions per unit of output compared to modern zig-zag kiln technology.

Steel, cement and foundries. High-temperature process industries contribute both combustion emissions and process-specific particulate matter (e.g., cement kiln dust), often concentrated in industrial belts on city peripheries.

Captive and backup power (diesel generators). Less visible than a power plant chimney but far more numerous, diesel generators used by commercial buildings, hospitals, telecom towers and factories as backup power are a distributed but significant urban emission source — especially during grid outages, when hundreds of DG sets across a city can fire up simultaneously. This is precisely why retrofitting them with certified Retrofit Emission Control Devices has moved from “good practice” to a compliance requirement in Delhi-NCR under GRAP rules — a small industrial source, multiplied across thousands of buildings, adds up. Our guide on how to reduce smoke from DG sets covers the practical side of this.

Regulation: what’s actually being done

India’s principal tool for controlling large industrial point sources is the Continuous Emission Monitoring System (CEMS) mandate, which requires designated “red category” industries (power plants, cement, steel, chemicals, and similar high-emission sectors) to install real-time stack monitors that feed data directly to State Pollution Control Boards and the CPCB’s central dashboard.

Other major levers include:

Flue-gas desulphurisation (FGD) requirements for coal power plants, aimed specifically at cutting SO₂
Zig-zag kiln conversion mandates for brick kilns, which studies show can cut particulate emissions by 20–30% compared to traditional fixed-chimney designs for a similar output
Emission norms under the Environment (Protection) Act, tightened periodically for specific industrial categories
The Commission for Air Quality Management (CAQM) in the National Capital Region, which coordinates cross-state industrial emission enforcement — recognising that airsheds don’t respect state boundaries
DG set emission norms, covering the distributed captive-power source described above; see our detailed look at DG set emission regulations

Enforcement remains the weak link more often than the rules themselves. Monitoring gaps, inconsistent inspection capacity, and the sheer number of small and medium industrial units operating outside formal monitoring networks mean that on-paper compliance and on-ground emissions don’t always match — a gap that shows up clearly in India’s National Clean Air Programme progress reports, which flag industrial emissions as one of the least-funded and least-monitored source categories nationally.

What this means if you manage an industrial or commercial facility

If your facility falls under CEMS requirements, compliance isn’t optional — but even outside that bracket, the same underlying physics applies at smaller scale. A poorly maintained furnace, an uncontrolled backup generator, or unmanaged fugitive dust from material handling all contribute to the same secondary-particulate chemistry described above, just at a smaller volume. Facility-level actions that move the needle include:

Ensuring combustion equipment (boilers, furnaces, DG sets) is properly tuned and maintained — poor combustion efficiency disproportionately increases particulate and CO output
Installing certified emission-control retrofits on diesel generators rather than treating them as a one-time compliance box to tick
Managing fugitive dust from material storage and handling, which behaves similarly to the construction dust sources covered elsewhere on this site
Tracking local AQI and understanding how PM2.5 differs from PM10 when interpreting monitoring data, since the two behave very differently in terms of source and health impact
FAQs

Is industrial pollution worse than vehicle pollution in Indian cities? It depends heavily on the city and the study methodology — estimates for both sources vary widely across research. In most Indo-Gangetic Plain cities, industry and transport are consistently among the top two or three contributors, but which one ranks higher shifts by city, season and study.

Can industrial emissions from one state affect air quality in another? Yes. Once SO₂ and NOₓ convert into secondary particulate matter, that particulate matter can travel well beyond the state or even country where it originated, which is why cross-border and cross-state airshed coordination has become central to India’s clean-air policy.

Do industrial emissions only matter in winter? No — the underlying emissions are often fairly constant through the year. What changes seasonally is the atmosphere’s ability to disperse them; winter’s still air and temperature inversions trap the same emission load closer to the ground, making it more concentrated and more visible.

What’s the single most effective industrial control measure? There isn’t one silver bullet — FGD for power plants, kiln modernisation for brick manufacturing, and CEMS-based monitoring for large point sources each target a different part of the problem. The most effective approach combines source-specific technology upgrades with consistent enforcement, which remains the harder half of the equation.