November 1, 2026
Posted by: editor
Category: RECD & Emission Control Devices

Every winter the same question comes up in building WhatsApp groups and facility review meetings across Delhi NCR. The generator next door is smoking, residents and staff are complaining, and someone suggests buying a few good air purifiers so the problem goes away. It sounds cheap, quick and sensible. In the air purifier vs RECD debate, though, the two devices are not competing for the same job.

The short answer: no, a room air purifier cannot replace treatment of DG exhaust. A purifier cleans some of the particles from a room’s air after the pollution has already left the stack, spread through the neighbourhood and leaked indoors. A retrofit emission control device (RECD) treats the exhaust at the source, in the pipe, before it is released. Regulators also judge compliance on what comes out of the DG set, not on how clean your conference room is.

Below, we put a generator’s exhaust flow next to a purifier’s clean air delivery rate, show the arithmetic, and then set out what a building near a DG set should do instead.

The Direct Answer in Three Sentences

Different problem: an RECD captures particulate matter inside the exhaust stream at high concentration. A purifier captures particles from room air after they have been diluted thousands of times over.
Different scale: a single mid-sized DG set pushes out more gas per hour than several household purifiers can circulate, and the diluted plume it creates is far larger again.
Different rulebook: State Pollution Control Board (SPCB) consents and CAQM directions in Delhi NCR apply to the generator’s emissions. A purifier inside a building does not count as emission control for the DG set.

Air Purifier vs RECD: The Scale Problem (CADR vs Exhaust Flow)

Both devices are rated in cubic metres per hour (m³/h), so the comparison looks simple. The units match; what they describe does not.

How much gas does a DG set actually push out?

Exhaust flow depends on engine displacement, aspiration (turbocharged or naturally aspirated), load and exhaust temperature. The only reliable figure for your engine is the one on the OEM data sheet. Our guide on using OEM exhaust flow data to specify an RECD explains where to find it and which conditions it is quoted at.

For the arithmetic here we use an illustrative figure. A 250 kVA diesel set at or near full load typically discharges something in the region of 1,500 to 3,000 m³/h of exhaust gas measured at exhaust conditions, i.e. hot gas at several hundred degrees Celsius. We will use 2,000 m³/h as a round illustrative value. Your engine may be higher or lower; always check the OEM figure, especially if you are sizing equipment for a specific unit such as a 250 kVA DG set.

How much air does a room purifier clean?

Portable purifiers are rated by clean air delivery rate (CADR). The US EPA’s guidance on air cleaners and air filters in the home puts source control and ventilation first and treats filtration as a supplement; its home air cleaner guide describes CADR as a measure of particle removal, usually measured at the highest fan speed, and uses it for sizing a unit to a room. Typical domestic and small-office units sold in India state CADR values in the range of roughly 300 to 500 m³/h. We will use 400 m³/h as the illustrative purifier.

Two points about CADR matter here. First, it is a particle metric only; it says nothing about gases. Second, it is measured in a test chamber at room temperature with well-mixed air, not in a stream of hot engine exhaust.

The arithmetic, done honestly

Illustrative step 1: matching volume flow on paper.

DG exhaust flow (illustrative): 2,000 m³/h
Purifier CADR (illustrative): 400 m³/h
Purifiers needed to “match” the volume: 2,000 ÷ 400 = 5 purifiers

Five purifiers sounds manageable, and this is usually where the “let’s just buy purifiers” argument stops. It should not, for three reasons.

Illustrative step 2: the gas is hot.

Exhaust at the manifold or stack outlet is typically several hundred degrees Celsius. A household purifier’s plastic housing, motor and pleated filter are designed for room air, so you cannot connect one to an exhaust pipe. Once the gas cools to room temperature its volume shrinks. Taking an illustrative exhaust temperature of 480 °C (753 K) and cooling to 25 °C (298 K), the volume scales by 298 ÷ 753 ≈ 0.40. So 2,000 m³/h of hot gas becomes about 2,000 × 0.40 = roughly 800 m³/h at room temperature. That looks easier still, at two purifiers. In practice the exhaust never reaches a purifier as a neat 800 m³/h stream, as the next step shows.

Illustrative step 3: the plume is diluted before it reaches anyone.

Once exhaust leaves the stack it mixes with outdoor air. By the time it drifts through a window or a fresh-air intake, it may be diluted by a factor of a hundred, a thousand or more. The pollutant mass is the same; it is simply spread through a much larger volume of air. To capture the same mass of particles from diluted air, a purifier must process that whole diluted volume.

If the 800 m³/h of cooled exhaust is diluted 100 times: 800 × 100 = 80,000 m³/h of polluted air
Purifiers at 400 m³/h needed to process that: 80,000 ÷ 400 = 200 purifiers
At 1,000-times dilution: 800 × 1,000 = 800,000 m³/h, or 800,000 ÷ 400 = 2,000 purifiers

These dilution factors are illustrative; real values vary hugely with wind, stack height, building geometry and distance. The direction is not in doubt, though. Treating pollution after it has spread means handling vastly more air than treating it in the pipe. Our post on why dilution misleads before-and-after measurements covers the same effect from the testing side.

Why even 200 purifiers would not solve it

Suppose a building did run hundreds of purifiers. It would still not have solved the problem:

Purifiers only clean the air that passes through them. Exhaust that disperses over the street, the neighbouring society or the school next door is untouched.
Gases pass straight through particle filters. Carbon monoxide, nitrogen oxides and most hydrocarbons are not captured by HEPA media (see the next section).
The DG set’s emissions are unchanged. From a regulator’s point of view, the stack is emitting exactly what it did before.

What a Purifier Can and Cannot Remove from Generator Smoke

Diesel exhaust is a mixture of solid particles (largely black carbon soot), condensed organic material, sulphates and water, along with gases such as CO, NO, NO2, CO2 and unburned hydrocarbons. A room purifier handles each component differently.

Exhaust component	Can a typical room purifier remove it?	Notes
PM2.5 (fine soot and condensed particles)	Yes, if it has a true HEPA filter and is sized correctly	Removes particles from the room air that passes through it; cannot touch particles still outdoors
Ultrafine particles (below about 0.1 µm)	Partly	HEPA media capture very small particles well by diffusion, but real-room performance depends on airflow, leakage around the filter and how fast new particles enter
Carbon monoxide (CO)	No	CO is a small, unreactive gas that passes through particle and standard carbon filters; only ventilation and source control help
NO and NO2 (NOx)	No, in practice	Some specialised sorbent or impregnated media adsorb NO2, but capacity is limited and NO is poorly captured; household units are not designed for this
Hydrocarbons and odour compounds	Limited	Activated carbon can adsorb some odour-causing organics, but a thin carbon layer saturates quickly under continuous load
CO2	No	Not removed by any household purifier
Heat and oxygen depletion	No	Irrelevant to a purifier’s function; these are reasons exhaust must never enter occupied spaces

The EPA’s guide notes that most portable cleaners use one filter type for particles and a separate one for gases. There are no widely used performance ratings for gas-removal filters, and activated carbon performs better when there is a lot of it. A purifier with a thin carbon sheet behind the HEPA layer is a particle cleaner with a small odour allowance, not a gas treatment system.

Does a HEPA purifier remove diesel fumes?

It removes the particle portion of diesel fumes from the air it processes, which is useful for occupant comfort. It does not remove the gaseous portion, and the word “fumes” is often used for exactly that part, the smell and the sting. If a room still smells of diesel with a purifier running at full speed, that is the gas phase passing through. For the source-side view of smell, see our post on whether an RECD can reduce diesel exhaust odour.

Does an air purifier remove carbon monoxide?

No. This is the most important line in the table, because CO is the exhaust component most likely to cause acute harm indoors. CO cannot be seen or smelled, and a purifier display showing “good” air quality says nothing about it, because most consumer purifier sensors measure particles (and sometimes total VOCs), not CO. Only a dedicated CO alarm detects it.

An RECD does not make CO safe either. Where an RECD design includes an oxidation catalyst, it can convert a share of CO and hydrocarbons once the catalyst is hot enough. Conversion is low during cold starts and at light load, and no device removes CO completely. Our explainer on RECDs, carbon monoxide and unburned hydrocarbons covers the light-off behaviour, and the NOx side is covered in do RECDs reduce NOx?. The answer there is that total NOx is essentially unchanged by filters and oxidation catalysts.

What about ultrafine particles?

Ultrafine particles are numerous and slip indoors easily; a good HEPA purifier captures a meaningful share of those that reach it. At the source, a wall-flow diesel particulate filter is very effective at capturing solid ultrafine particles in the exhaust stream. Our batch post on RECDs and ultrafine particles in DG exhaust explains capture mechanisms and why partial-flow designs perform differently.

The Compliance Boundary: Purifiers Do Not Count

This is where the purifier idea fails on practical as well as technical grounds.

Emission rules for DG sets in India apply to the engine and its exhaust. CPCB has issued a procedure for emission compliance testing of retrofit emission control devices for in-use DG engines up to 800 kW gross mechanical power. Type-approved RECDs must show at least 70% particulate matter capture under that procedure, with testing by recognised agencies such as ARAI and ICAT. SPCBs set consent conditions for DG sets, including stack height. In Delhi NCR, CAQM directions govern when and how DG sets may run, including during GRAP stages.

None of those frameworks looks at indoor room air as a proxy for the generator’s emissions. An inspector checks the DG set: its emission control status, documentation, stack and consent conditions. A receipt for twelve air purifiers is not evidence of emission control, and a generator that should have an RECD but does not is exposed to the same risks of sealing and penalties either way.

If you need the regulatory background, start with why RECDs are mandatory in India and our summary of GRAP rules for DG sets in Delhi NCR. Requirements vary by state, sector, capacity and consent, so confirm your own position with your SPCB and with the Central Pollution Control Board notifications current at the time you read this.

“But our AQI monitor shows clean air indoors”

Good indoor readings are good news for occupants, but they are not evidence about the stack: a PM monitor measures diluted room air at one point and moment. Treat them as a measure of occupant exposure, not of DG emissions.

Safety First: Exhaust Must Never Be Routed into Occupied Rooms

Sometimes the idea becomes: “bring the exhaust inside and run it through filters.” Never do this.

Carbon monoxide passes through both purifiers and particulate filters. Concentrations in undiluted or lightly diluted exhaust can rise to dangerous levels within minutes in an enclosed space.
NO2 and other irritant gases remain.
Heat and fire risk: exhaust is hot enough to damage equipment, ignite combustible material and overheat rooms.
Oxygen depletion is possible in enclosed spaces where combustion products displace air.

The US CDC’s guidance on carbon monoxide is blunt: never run a generator inside a home or garage, even with doors and windows open. It also advises battery-powered or battery-backup CO alarms. Indian sites should apply the same principle. DG exhaust must discharge outdoors through a properly designed stack that follows the engine OEM’s requirements, local building and fire NOC conditions, and SPCB stack-height conditions.

An RECD does not change this. It reduces particulate emissions; it does not make a generator safe to operate indoors or make exhaust safe to breathe. Our sister post, does an RECD make a diesel generator safe to operate indoors?, deals with this question directly.

Install CO alarms where exhaust could reach people

For rooms adjacent to DG rooms, basements with DG sets, lift lobbies near stacks and any space where exhaust has been smelled, fit CO alarms that meet a recognised standard. Test them regularly and replace them according to the maker’s instructions. A CO alarm is cheap, it measures the gas a purifier ignores, and it warns occupants when something has gone wrong with the stack, a joint or the ventilation.

Where Air Purifiers Genuinely Help

Protecting occupants from outdoor particles that leak in

In Delhi NCR winters, outdoor PM2.5 comes from traffic, dust, biomass burning, industry and generators together. For bedrooms, a crèche, a clinic waiting area or a meeting room, a correctly sized HEPA unit can noticeably cut the PM2.5 people breathe.

Bridging the period before source control is in place

If a DG set is awaiting an RECD order, purifiers in the most affected rooms can reduce particle exposure in the meantime, as a temporary comfort measure with CO alarms alongside. If budget is the worry, compare honestly: a purifier fleet buys cleaner air in a few rooms, while an RECD buys source control and compliance.

Complementing, not substituting

The right mental model is two layers:

Layer 1, source control: the DG set’s emissions are reduced at the stack, the stack is correctly sited and the generator is maintained. This is what compliance is built on.
Layer 2, room cleaning: purifiers reduce the residual particles that still reach occupied rooms from all outdoor sources.

Layer 2 without Layer 1 is like mopping the floor with the tap still running.

Source Control vs Room Cleaning: A Side-by-Side View

Question	Room air purifier	RECD at the DG set
Where does it act?	Inside a single room, on diluted air	In the exhaust line, on concentrated exhaust
What does it mainly capture?	Particles in room air (HEPA); limited odour (carbon)	Particulate matter in exhaust; some designs also oxidise part of the CO and HC once hot
Does it remove CO?	No	Partly, only where the design has an active oxidation catalyst at temperature; never fully
Does it reduce total NOx?	No	No, not materially (filters and oxidation catalysts do not reduce total NOx)
Does it protect neighbours?	No, only the room it is in	Yes, by reducing particulate leaving the stack
Does it count for SPCB/CAQM compliance?	No	Yes, when it is a type-approved device correctly installed and maintained
Typical scale	Hundreds of m³/h of room air	Full engine exhaust flow (illustratively thousands of m³/h of hot gas)

What to Do Instead: A Plan for Buildings Near a DG Set

If your office, shop or society sits close to a DG set (your own or a neighbour’s), work through these steps in order.

Identify the source and how exhaust reaches you. Note when the smell or smoke appears, the wind direction and which windows, vents or intakes are affected. If the DG set belongs to someone else, a methodical log is more persuasive than anecdotes. Our guide to investigating air pollution complaints after an RECD installation sets out a respectful, evidence-based approach that works before installation too.
Check the stack location and height. A stack that discharges below the roofline of a neighbouring building, or into a courtyard, will cause problems whatever is fitted to it. Stack height must meet SPCB consent conditions. Raising a stack can improve dispersion, but it does not reduce emissions; see is increasing chimney height an alternative to an RECD?.
Separate exhaust from air intakes. Building fresh-air intakes, AHU louvres and openable windows near the stack pull exhaust indoors. Review this with your HVAC consultant and read our post on RECD installation near fresh-air intakes.
Fit a type-approved RECD on the DG set. This is the source-control step that addresses particulate emissions and compliance. Specify it from the OEM exhaust data, load profile and site conditions.
Maintain the engine and the RECD. A badly tuned engine that smokes heavily loads any after-treatment faster and smells worse. Follow the service schedule in our RECD maintenance guide.
Install CO alarms in rooms near the DG room, in basements and anywhere exhaust has been noticed.
Then add purifiers to occupied rooms where outdoor particle levels remain a concern, sized by CADR to the room volume.
Verify with appropriate monitoring. For workplaces, structured exposure assessment is more useful than spot readings. Our sister post on RECDs and workplace air monitoring explains how to assess exposure changes properly.

RWAs with their own DG sets can use our RECD purchase planning guide for housing societies.

Worked Example: An Office Floor Next to a DG Stack

The figures below are illustrative and chosen to make the arithmetic easy to follow. Substitute your own measurements.

The space

Floor area: 500 m²
Ceiling height: 3 m
Room volume: 500 × 3 = 1,500 m³

One purifier

Purifier CADR: 400 m³/h
Clean air changes per hour (ACH) from one unit: 400 ÷ 1,500 = 0.27 ACH (to two decimal places)

At 0.27 ACH, one purifier processes a volume equal to the whole floor roughly once every 1,500 ÷ 400 = 3.75 hours. For an open-plan office that is far too slow to have a meaningful effect.

Enough purifiers for a useful rate

Many indoor air guides use roughly 5 clean air changes per hour from filtration as a practical target for occupied rooms; check current guidance for your building type. We will use 5 ACH as an illustrative target.

Required clean air delivery: 1,500 × 5 = 7,500 m³/h
Purifiers at 400 m³/h: 7,500 ÷ 400 = 18.75, so 19 purifiers

Nineteen units is a large commitment in power, filters, floor space and noise, just to keep particles down on one floor.

The open-window scenario

Now suppose a window on the side facing the DG stack is left open, or the AHU intake sits near the stack, while the generator runs during a power cut. Exhaust that has been only modestly diluted can enter continuously. In qualitative terms:

Particles: the 19 purifiers fight a continuous fresh supply of soot, and indoor PM2.5 can stay high while the window is open.
CO and NO2: these accumulate, because the ventilation path is itself the source. Purifier displays may still read “good”.
Odour: thin carbon layers saturate, and the diesel smell persists.

Compare that with the source side. The illustrative DG set produces about 2,000 m³/h of hot exhaust, roughly 800 m³/h once cooled. An RECD in the exhaust line deals with that stream before any dilution, at one location, for every hour the generator runs. The office still needs the window closed, the intake relocated and a CO alarm fitted. But the particulate leaving the stack is reduced at the point where it is most concentrated and easiest to capture.

The lesson: purifier counts scale with room volume, not with the DG set, and no number of them addresses the gases. Fixing the pathway and the source does more than any room-level measure.

FAQ

Can an air purifier remove generator smoke from a room?

A HEPA purifier can remove a good share of the smoke particles from the air that passes through it, if it is sized correctly for the room. It does not remove the gases in generator exhaust, such as carbon monoxide and nitrogen oxides, and it does nothing about smoke outdoors.

In the air purifier vs RECD comparison, which one satisfies pollution control rules?

Only the RECD. SPCB consent conditions and CAQM directions in Delhi NCR apply to the DG set’s emissions. Indoor purifiers are not recognised as emission control for a generator. Confirm your specific obligations with your SPCB.

How many purifiers would match a DG set’s exhaust flow?

On paper, an illustrative 2,000 m³/h exhaust flow divided by a 400 m³/h purifier CADR is 5 units. That comparison is misleading, because exhaust is hot and contains gases, and once diluted outdoors it occupies a volume hundreds or thousands of times larger. Check your engine’s actual flow on the OEM data sheet.

Does a purifier protect against carbon monoxide from a DG set?

No. Purifiers do not remove CO, and most purifier sensors cannot detect it. Fit CO alarms in rooms near DG sets and make sure exhaust discharges outdoors through a properly designed stack.

Is it ever acceptable to route DG exhaust indoors through filters?

No. Neither purifiers nor RECDs make exhaust safe to release into an occupied space. CO, NO2, heat and fire risk remain. Exhaust must go outdoors via a stack designed to OEM, fire NOC and SPCB requirements.

Start with the Stack, Then Clean the Rooms

If your building is near a DG set, the improvement starts at the exhaust pipe. Aceget supplies CPCB type-approved retrofit emission control devices for DG sets from 25 kVA to 2500 kVA. Aceget’s product page describes its RECD as using a silicon carbide ceramic filter with a precious-metal coating, an oxidation catalyst stage and NO2-assisted regeneration, with up to about 96% particulate reduction depending on DG condition and fuel. You can review our type-approval documents on the certifications page. Share your DG nameplate, OEM exhaust data and site layout through our contact page, and our team will help you plan source control, so that any purifiers you buy are cleaning the rooms they were meant for, not making up for an untreated stack.

Can a Room Air Purifier Replace Treatment of DG Exhaust?