It extinguishes by displacing oxygen, at concentrations that will kill anyone in the room.

That is the mechanism rather than a side effect, so it cannot be designed around.

Clean agents and inert gases work at concentrations designed to be survivable for brief exposure.

CO2 does not, which restricts it to normally unoccupied enclosures and machinery.

People have been killed both by fire discharges and by accidental discharges during maintenance.

Where a space may be occupied, a different agent is the correct choice.

Normally unoccupied enclosures and machinery - presses, machine tools, dust collectors, generator enclosures and marine machinery spaces.

It is effective on flammable liquids and energised electrical equipment.

It leaves no residue at all, which suits machinery that would be damaged by powder.

It is inexpensive compared with clean agents and not subject to the same environmental restrictions.

Local application to a specific machine is possible rather than flooding a whole room.

In each case the space is one where people are not expected to be present.

Pre-discharge alarms, extended time delays, pneumatic sirens, lock-off devices, warning signage and entry procedures.

The delay has to be long enough for anyone present to leave the space.

Pneumatic sirens work without electrical power, which matters during a fire.

Lock-off devices prevent discharge while maintenance work is in progress.

Signage at every entrance warns anyone approaching.

The specific requirements differ between markets but the principle is universal.

The atmosphere remains lethal long after the fire is out, and CO2 is odourless, colourless and heavier than air.

It collects at low level and in pits, basements and bilges.

There is no sensory warning that a space is not survivable.

Ventilation and atmospheric testing before entry are not optional.

Rescue attempts into CO2-filled spaces are a recurring way a single fatality becomes several.

Entry procedures should be established and rehearsed before a discharge ever occurs.

One that discharges at a specific machine or hazard rather than flooding an enclosure.

Nozzles are aimed at the equipment, so less agent is needed.

It suits machinery in a larger space that cannot practically be flooded.

The hazard is reduced but not eliminated, because dangerous concentrations still occur locally.

Alarms, delays and signage requirements still apply.

Design is specific to the machine geometry, so moving or replacing equipment invalidates it.

Not in the way clean agents are - CO2 is not subject to the same phase-down pressures.

That is one of the reasons it remains in use where the space permits.

Recharge is inexpensive and the agent is widely available.

The regulatory attention on CO2 systems is about personnel safety rather than environmental impact.

Some jurisdictions have tightened requirements following fatalities.

Those requirements should be checked locally, as they have changed in several markets.

Cylinder weighing or level checking, pressure testing, detection and release testing, and verifying every safety device.

Agent quantity is normally verified by weight, since CO2 is stored as a liquid.

Cylinders fall under pressure equipment rules with their own inspection intervals.

Lock-off devices must be proven to work, since maintenance safety depends on them.

Alarms, sirens and delays are tested functionally rather than assumed.

Maintenance itself is the activity where accidental discharges have historically killed people, so the procedure matters as much as the schedule.