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The Science

How our purifiers and shower filters actually work — no magic claimed.

The honest version of how this stuff works

Most product pages tell you what to buy. This one tells you why it works — and where it stops working. We think you should understand the physics before you spend ₹6,000 or ₹26,000, because the science is genuinely simple once someone explains it without the marketing fog.

There is no magic here. An air purifier moves room air through filters and pushes cleaner air back out. A shower filter passes your tap water through a stack of media that reduces the things that damage hair, skin and your geyser. Both are mechanical. Both have honest limits. We will name those limits plainly, because a brand that hides them is a brand that is hoping you do not read closely.

Use the two tabs below. The first covers air — HEPA, carbon, and the often-misunderstood CADR number. The second covers water — hardness, the cartridge stages, and exactly where a point-of-use shower filter helps versus where you genuinely need a whole-house softener instead.

How HEPA filtration works

Air goes through three stages, in order

A Girnaar purifier pulls room air in with a fan and passes it through filter layers stacked one behind the other. Each layer has a different job, and the order matters.

  1. Pre-filter — a washable mesh that catches the big, visible stuff: hair, lint, pet fur, large dust. This is the bouncer at the door. It exists mainly to protect the expensive HEPA layer behind it, so the HEPA does not clog with debris it was never meant to handle. You vacuum or rinse this one; you do not replace it often.
  2. HEPA H13 filter — a dense mat of randomly arranged glass-fibre threads. This is where the fine particles are captured: smoke, the PM2.5 that AQI apps report, pollen, fine construction dust. More on the physics below.
  3. Activated carbon — a bed of treated carbon, full of microscopic pores, that grabs gases and smells the HEPA layer lets straight through. Cooking odours, paint and new-furniture fumes, the chemical edge in city traffic air.

Diagram of air moving through pre-filter, HEPA H13 and activated carbon stages

Why 0.3 µm is the hardest particle to catch

Here is the part most people get backwards. People assume the tiniest particles are the hardest to capture. They are not. A HEPA filter is not a sieve with holes — its fibres catch particles using three different mechanisms at once, and each works best on a different size.

  • Impaction — big, heavy particles have too much momentum to follow the air as it curves around a fibre. They carry straight on and slam into it. This catches the large particles easily.
  • Interception — mid-size particles follow the airstream, but they are wide enough that an edge brushes a fibre as they pass, and they stick. This catches the medium range.
  • Diffusion — the very smallest particles are so light they get knocked around randomly by air molecules (Brownian motion), zig-zagging instead of flowing smoothly. All that wandering makes them very likely to bump into a fibre. This catches the ultrafine particles.

Notice the gap. Impaction and interception are strong for large particles. Diffusion is strong for ultrafine ones. In between sits a size where a particle is too small to impact reliably and too big to wander much — so all three mechanisms are at their weakest at once. That size is around 0.3 micrometres, and engineers call it the Most Penetrating Particle Size (MPPS). It is, quite literally, the hardest particle for a HEPA filter to catch.

So filters are rated at their worst case. Under the European standard EN 1822, a HEPA H13 filter is certified to capture at least 99.95% of particles at the MPPS — that 0.3 µm worst case. Because it does that well at the hardest size, it does even better on everything larger and everything smaller. When you see “99.95% at 0.3 µm”, that is the standard talking, not a marketing flourish.

CADR: how fast it actually cleans your room

A great filter is only half the story. The other half is how much air the machine pushes through that filter every hour. That is what CADR measures — Clean Air Delivery Rate, usually given in cubic metres per hour (m³/hr). A high CADR means more clean air, faster.

Think of it with a worked example. Take a 12×12 ft bedroom — that is 144 sq ft. With a standard 10 ft ceiling, the room holds roughly 40 cubic metres of air. If a purifier had a CADR of 200 m³/hr, then in one hour it could process about five roomfuls of air (200 ÷ 40 = 5). That is what people mean by “air changes per hour” — and for a noticeable difference in a polluted Indian city, you generally want a purifier whose CADR comfortably exceeds your room volume several times over.

This is why a purifier sized for a 144 sq ft bedroom struggles in a 400 sq ft living room — same machine, much bigger volume, far fewer air changes per hour. Match the CADR to the room, not the other way around. Girnaar’s specific CADR ratings are on each product — Girnaar Breeze is rated at [SPEC_PLACEHOLDER] m³/hr and Girnaar Summit at [SPEC_PLACEHOLDER] m³/hr. We would rather you matched them honestly than oversold you a number.

What the carbon stage adds

HEPA catches particles. It does almost nothing for gases and smells, because a gas molecule is far smaller than any fibre mat can hold. That is the carbon’s job. Activated carbon is riddled with pores so fine that a single gram has an enormous internal surface area, and gas molecules stick to that surface — a process called adsorption (they cling to the surface, rather than being soaked in). It is what reduces:

  • Cooking odours and that lingering tadka smell after dinner;
  • VOCs — volatile organic compounds — the fumes off fresh paint, new furniture, adhesives and cleaning products;
  • The general chemical staleness of a closed-up flat near heavy traffic.

Carbon has a finite capacity. Once its pores fill up, it stops working, which is why the filter is a replaceable part and not a lifetime fitting.

What a purifier cannot do

Being honest builds trust, so here is the plain list of what a purifier is not.

  • It does not make oxygen. A purifier cleans the air already in your room; it does not generate or add oxygen. Anyone selling you an “oxygen-boosting” purifier is selling you a story.
  • It cannot fix outdoor air. It works on the air inside a reasonably closed room. Open every window onto a smoggy Delhi winter evening and it is fighting an ocean with a bucket. It is an indoor tool for an indoor space.
  • It is not a medical device. Many people report breathing more comfortably in a cleaner room, and reducing indoor particles and allergens may help — but if you have asthma, allergies or any breathing condition, please treat a purifier as a comfort aid alongside your doctor’s advice, never as a treatment.

Want help matching CADR to your actual room? Take two minutes with Find Your Purifier, read the full air purifier buying guide, or browse the air purifiers themselves.

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