The Association of Secondary Teachers in Ireland has expressed "disappointment and dismay" at confirmation that the delivery of 10,000 C02 monitors to schools will be delayed for some weeks.

The ASTI union said the monitors constitute an important extra tool in the fight to ensure that Covid-19 transmission is minimised in schools and it is "disheartening in the extreme" that the roll-out has been delayed.

The Teachers' Union of Ireland described the delay as "extremely disappointing".

It said: "We have made strong representation to the Department of Education today that the outstanding allocation of these monitors must be delivered to schools as soon as is possible."

The delivery of C02 monitors to schools was scheduled to be completed by early next week.

The delay is due to a fault discovered in some of the yet-to-be delivered devices according to Lennox, the company tasked with their supply and distribution.

Lennox said the delay has arisen because a component used within the monitors failed the required quality control check. It said the fault was with the LCD display unit in the C02 devices.

The monitors measure air quality in a room and serve as an indicator as to when windows and doors should be opened to improve ventilation.

The issue is mainly affecting second level schools.

Lennox said that every post primary school will have received 10 C02 monitors by the end of this week.

This leaves a balance of between 10 and 25 units per school to be provided, depending on the school size.

At primary level, Lennox said that 96% of schools will have received their full allocation by the end of this week.

The remainder – all medium to large schools – will continue to wait for between three and 10 monitors each. All those schools will have received 10 monitors each by the end of this week, the company said.

According to Lennox, it was informed by the manufacturer that there would be a delay with the delivery of the last batch of C02 monitors, because of a fault detected which meant the display would not have operated when assembled.

It said quality assurance found no similar concerns with any of the 25,000 C02 Monitors that have already being distributed to schools.

It added that it is working with the manufacturer to confirm how quickly a replacement batch of the 10,000 affected monitors can be provided to schools.

"Given the global demand for C02 monitors, this is expected to be during October," a statement said.