It saponifies fats into soluble soap and breaks down protein material such as hair, generating heat that helps melt congealed grease.

Sodium and potassium hydroxide react with fats and oils in a saponification reaction, converting them into soap and glycerol. Both are water-soluble and flush away, so the blockage is not pushed along the pipe but removed.

The same alkalinity hydrolyses hair and other protein material, which is why caustics are effective on the mixed hair-and-soap blockages typical of shower and basin wastes.

Dissolution of the hydroxide in water is exothermic, and the reaction with fat generates further heat. That warmth softens congealed grease and speeds everything up.

The heat is also the hazard. In a fully blocked pipe with standing water it can become hot enough to damage plastic pipe and to cause a violent release if the blockage clears suddenly.

Solid obstructions, root intrusion, collapsed pipework and mineral scale - none of which is affected by alkalinity.

An object dropped into the drain, a build-up of wipes, or a collapsed section will not dissolve. Adding chemical simply leaves a pipe full of caustic solution that then has to be dealt with by whoever clears it mechanically.

Roots are woody and are not removed by caustic products in any practical timeframe; they require cutting and a repair to the defect that admitted them.

Mineral scale and limescale are addressed by acid, not alkali.

Recurring blockages in the same place usually indicate a defect - a back-fall, a displaced joint, an undersized pipe - and chemistry becomes a monthly ritual rather than a solution.

If a first correct dose does not work, escalate to mechanical clearance or a camera survey rather than a second dose.

Generally safe on modern plastic and cast iron in normal use, but heat and prolonged contact can damage plastics, and it attacks aluminium.

Modern plastic waste pipe tolerates correct use, but caustic left standing in a fully blocked pipe generates sustained heat that can soften joints and deform thin-walled components such as traps and waste fittings.

Aluminium reacts with strong alkali, releasing hydrogen. Aluminium components in a waste system are uncommon but do exist, and the combination should be avoided.

Older pipework, degraded plastics and poorly made solvent joints are more vulnerable than new material.

Chrome and decorative finishes on visible waste fittings are frequently damaged by splashes, which is a common and avoidable complaint.

Follow the dose and dwell time on the label rather than leaving product in overnight in the hope of a better result - extended contact is where material damage happens.

Chemical splash goggles, chemically resistant gloves and covered skin - not safety glasses, because the eye injuries caused by caustic splash are severe and permanent.

Caustic solutions cause deep, progressive burns that may not be immediately painful, so exposure can be more serious than it first appears.

Eye protection must seal against splashes. Open-sided safety glasses do not, and the characteristic injury comes from splash-back out of the drain.

Gloves must be a material rated for alkalis, with sleeves covering the wrist and forearm.

Pour slowly and keep the face away from the opening. Never lean over a drain that has been dosed, and never use a plunger afterwards - plunging can eject caustic solution upward.

Have running water available for irrigation, and know the first-aid procedure: prolonged flushing with water and immediate medical attention for any eye exposure.

They react violently, generating heat and gas, and the reaction can eject hot corrosive liquid out of the drain.

Acid and alkali neutralise each other exothermically. In a confined, partially blocked pipe with standing water, that release of heat can boil the liquid and force it back up the way it came.

Depending on the products involved, hazardous gases may also be produced.

The dangerous scenario is sequential rather than deliberate: one product is tried, fails, and a different type is poured in afterwards. The first product is still there.

The rule is one chemistry per drain. If a product has failed, the drain must be cleared mechanically or flushed with large volumes of water by someone who knows what is already in it.

Anyone attending a drain that has been treated should be told what was used - this is a genuine hazard for plumbers and drainage engineers.

It is much less effective and considerably more hazardous - the product dilutes before it reaches the blockage and heat builds in the standing water.

Caustic works by contacting the blockage at strength. Standing water dilutes it on the way down, so a fully blocked, water-filled pipe is the worst case for effectiveness.

The heat generated stays in the standing water rather than dispersing, which is where pipe damage and eruption risk come from.

Products formulated as dense gels are designed for exactly this situation, sinking through standing water to reach the blockage at strength.

Where a sink is completely blocked and full, removing as much water as possible first improves both the result and the safety.

A completely blocked drain with standing water is generally a mechanical clearance job. Chemistry is better suited to slow-running drains and to preventive maintenance.

It is used that way, but enzyme maintainers are the better preventive product and caustic is better reserved for clearing.

Routine caustic dosing keeps grease from establishing in kitchen wastes, and some operations schedule it. It works.

The objections are cumulative: repeated heat and alkali exposure on pipework and joints, risk to staff performing a routine task with a hazardous chemical, and the discharge of caustic to the sewer.

Enzyme and bacterial maintainers are designed for scheduled preventive dosing. They are safe to handle, work overnight on the residual film in the pipe, and do not degrade pipework.

A common arrangement is enzyme maintainers on a regular schedule with caustic held in reserve for blockages.

Where discharge is subject to trade effluent consent, check whether regular caustic dosing is compatible with the pH limits in that consent.