Masking covers the smell, neutralisers bind or chemically alter the odour molecule, and enzymes digest the material producing it. Only enzymes address the cause.

Fragrance-based products give an immediate improvement that lasts as long as the fragrance does. They are useful before an event and useless as a solution.

Counteractants react with or encapsulate the odour molecules already in the air and on the surface. They work faster than enzymes and do not remove the source material.

Enzyme and bacterial products break down the proteins, urea and organic residues that generate odour continuously. They need time, moisture and a temperature range to work, and they must physically reach the contamination.

In practice a combined approach is common: enzyme treatment for the source, with a counteractant for immediate relief while the enzymes work.

Because urine penetrates below the carpet into backing, underlay and subfloor, crystallises as it dries, and rehydrates whenever humidity rises.

A urine deposit is far larger below the surface than the visible mark suggests - it spreads as it soaks and reaches the layers beneath.

As it dries, urine salts crystallise. Those crystals are not volatile when dry, so the smell fades; when humidity rises they take up moisture and the odour returns strongly. This is why the problem seems to come and go.

Surface cleaning removes the visible stain and almost none of the deposit.

Effective treatment means applying enzyme solution in enough volume to reach the full depth of contamination, keeping it wet for hours, then extracting. Severe or long-standing contamination may require lifting the carpet and treating or replacing the underlay, and sealing the subfloor.

An ultraviolet lamp in a darkened room reveals the true extent, which is usually considerably larger than expected.

Enough to reach the full depth of the contamination - which for a soaked-in spill means flooding rather than spraying.

The chemistry has to physically contact the material producing the odour. Where that material is in the backing and underlay, a surface mist never reaches it.

The general rule is to apply approximately the volume that was originally spilled, distributed over the affected area, so the solution penetrates to the same depth.

The area must stay wet for the enzyme's working period. Covering with plastic sheet prevents it drying out prematurely.

Over-application has its own risks - saturating a carpet promotes browning, delamination and secondary odour - so the treatment should be followed by extraction and forced drying.

For a large or valuable installation, treat a test area first and assess after it has fully dried, since the result cannot be judged while damp.

On organic odours - urine, vomit, milk, food, body fluids - yes. On smoke, chemical and damp odours, no.

Enzymes digest specific classes of organic molecule. Where the odour comes from that material, removing it removes the smell permanently.

Smoke odour is deposited as tar and particulate across every surface in a space, not just the carpet, and is addressed by cleaning all surfaces, sealing where necessary and often by specialist treatment.

Chemical odours from spills, solvents or off-gassing materials need the source removed or ventilated; enzymes have nothing to act on.

Damp and mould odours are a moisture problem. Treating the carpet without fixing the water ingress guarantees recurrence, and where microbial growth is established in the backing, replacement is often the honest answer.

Identify the source before choosing chemistry - most deodorizer failures are misdiagnoses.

For light, general freshening between cleans, yes. For a real odour source, no - and they can accumulate in the pile.

Sprinkle-and-vacuum powders adsorb some airborne odour and carry fragrance. They are cheap, easy and require no drying time, which makes them popular for routine housekeeping.

They do not reach or treat contamination in the backing, so they have no effect on urine, spills or microbial odour.

Residue is a genuine issue. Powder not fully vacuumed out builds up in the pile, can look like soiling, and in humid conditions may clump.

They also mask changes that should be investigated - a carpet that needs powdering weekly has a problem that powder is hiding.

Where powders are used, thorough vacuuming afterwards is essential, and they should not be used on damp carpet.

By finding and fixing the moisture first - no deodorizer works on carpet that keeps getting wet.

Musty odour indicates microbial activity, which requires moisture. Common sources are leaks, rising damp, condensation on a cold slab, poor drying after previous cleaning, and water ingress at thresholds.

Until the source is corrected, any treatment is temporary.

Once dry, the carpet may recover with cleaning and an appropriate antimicrobial treatment where permitted locally. Where growth is established in the backing or underlay, replacement of the affected material is usually the correct answer.

Underlay is often the reservoir even when the carpet itself appears sound, and assessing it means lifting a corner rather than judging from above.

Improving drying after routine cleaning prevents the problem recurring - over-wetting is a frequent cause of the very odour cleaning was meant to remove.

Most can, with ventilation and attention to fragrance sensitivity - and fragrance-free options exist for exactly that reason.

Enzyme and bacterial products are low-hazard and low-odour, and are widely used in occupied premises. Their limitation is the dwell time, which usually means treating outside working hours.

Strong fragrances cause complaints, particularly in healthcare, education and offices where some occupants report sensitivity. Unfragranced counteractants avoid the issue.

Any treatment that leaves carpet wet for hours needs the area closed off - both for slip risk and because walking on it defeats the treatment.

Check the safety data sheet for ventilation requirements and any restriction on use in occupied spaces.

Where an area cannot be closed, plan the work in sections so that part of the space remains usable.