When combined chlorine exceeds the limit, after heavy bather load or contamination, after heavy rain, and on a routine schedule in commercial operation.

The primary trigger is a combined chlorine reading above the level the applicable standard permits. That indicates chloramines have accumulated and routine dosing will not clear them.

Heavy use - a busy weekend, a swimming lesson programme, a party - loads the water with organic material faster than the residual can oxidise it.

Rain and wind bring in organic debris, phosphates and dilution, all of which raise demand.

Water that looks dull, smells sharply of chlorine, or irritates eyes is showing chloramine symptoms regardless of what the free chlorine test says.

Commercial operations usually shock on a schedule as well as on demand, because prevention is cheaper than recovering a pool that has been allowed to drift.

The point at which enough free chlorine is present to break the bond in chloramines and destroy them, rather than simply adding to them.

Chlorine added to water containing ammonia and nitrogen compounds first forms chloramines. Adding more initially increases combined chlorine.

Beyond a threshold - roughly several times the combined chlorine concentration - the chemistry changes and the chloramines are oxidised and destroyed, releasing nitrogen gas.

This is why a partial dose is worse than none: it raises combined chlorine and makes the smell and irritation worse without reaching the point where the problem clears.

In practice: measure combined chlorine and calculate a dose that will exceed breakpoint for the pool's volume, then verify afterwards that combined chlorine has fallen.

If it has not, the dose was insufficient and the pool is now in exactly the state the shock was meant to correct.

Chlorine shock to destroy chloramines and to disinfect; non-chlorine shock to oxidise organics and reopen quickly, but it does not achieve breakpoint.

Chlorine-based shock is the only option where combined chlorine must be destroyed or where a contamination event requires disinfection. It takes the pool out of use until levels fall.

Potassium monopersulfate shock oxidises organic material without adding chlorine and without raising the residual, so bathing can often resume very quickly.

It is genuinely useful for routine oxidation between chlorine shocks, particularly in spas and heavily used pools where downtime is expensive.

It does not reliably destroy chloramines, and it interferes with some combined chlorine test methods, which can produce a falsely alarming reading afterwards.

The usual arrangement is non-chlorine shock for routine oxidation and chlorine shock when the combined chlorine reading demands it.

From the pool volume and the measured combined chlorine, to reach a free chlorine concentration several times the combined level.

Pool volume must be known accurately. Estimating it is the commonest source of under-dosing, and an under-dose is the failure mode that makes things worse.

Measure combined chlorine first. The target free chlorine concentration is a multiple of it - product instructions and standard guidance give the factor.

Product strength varies substantially between chemistries, so the dose is calculated from the specific product's available chlorine, not by volume of product.

Round up rather than down. Reaching breakpoint matters; slight excess simply takes longer to fall.

Retest after circulation has distributed the dose and the reaction has had time, and confirm combined chlorine has fallen before declaring the job done.

Because ultraviolet light destroys free chlorine quickly, so a daytime shock on an outdoor pool is partly wasted before it works.

Sunlight degrades unstabilised chlorine within hours. A shock dose applied in the morning may fall below breakpoint before the reaction is complete.

Dosing at dusk gives the chemistry the whole night to work with no ultraviolet loss, and the level has usually fallen into the acceptable range by morning.

Circulation must run throughout. A shock dose sitting in one part of a static pool treats that part and nothing else, and high local concentrations can damage liners and finishes.

Indoor pools have no ultraviolet issue, so timing is set by occupancy instead - typically after the last session.

Keep bathers out until free chlorine has fallen to the normal operating range, and verify by test rather than by elapsed time.

Yes - undissolved granules bleach liners and painted finishes, and repeated high doses affect some surfaces and equipment.

Broadcasting granules onto a vinyl liner or a painted surface is the classic damage mechanism. Granules sink, sit on the surface and bleach a permanent pale patch.

Pre-dissolving in a bucket of pool water and pouring the solution around the perimeter with the circulation running avoids it entirely.

Very high concentrations can affect metal fittings, heat exchangers and some plastics, particularly if pH is allowed to fall at the same time.

Calcium hypochlorite raises calcium hardness with every shock, which in hard water accelerates scaling.

Balance the water before shocking rather than after. Shocking a pool with pH out of range is both less effective and harder on the surfaces.

Because a small dose creates more chloramines without reaching breakpoint - the smell is combined chlorine, not free chlorine.

The characteristic sharp swimming pool smell comes from chloramines. A well-run pool with adequate free chlorine and low combined chlorine has very little odour.

Adding a modest amount of chlorine to water already carrying nitrogen compounds produces more chloramines. The smell intensifies and eyes sting more.

The instinctive response - to reduce chlorine because the pool 'smells too chlorinated' - makes the problem considerably worse.

The correct response is to measure combined chlorine and shock to breakpoint.

Fresh water dilution also helps, and in heavily used pools regular replacement of a proportion of the water is part of the standard operating regime for exactly this reason.