Italian CO filter

Filtro aggiuntivo contro il CO
Additional filter against CO
Issued for	O.39 M.31-33
400 g circa
40mm DIN
Made in	Italy
Period	1939-1945 1945-?
Issued to	Artillerymen in unventilated fortifications
Protection level when new	CO
Manufacturer	Pirelli

The Italian military CO filter was developed and issued to those artillerymen that had were located in bunkers that lacked ventilation, usually older fortifications and smaller ones.

CO protection in the army

CO protection was needed in bunkers due to the heavy artillery use, which led to a build-up of said odourless gas that would kill the soldiers without them realizing. To account for this problem the biggest and/or more recent bunkers were equipped with a centralized ventilation system to which hosed masks could be attached, while the bunkers that couldn't host such a system (namely old and small fortifications) had their soldiers equipped with individual respiratory protection, which consisted of an O.39 mask with its kit and a CO filter.

Two O.39 masks connected to a pipe through which filtered air would pass

Description

The filter was made of aluminium painted blue, with a base diameter of 10 cm and a height of 8 cm. It contained a central layer of hopcalite that would filter the CO and 2 layers of calcium chloride that would make the hopcalite work more efficiently due to their hygroscopic capabilities. The layers were divided by riddled metal sheets, with the lowest one having a spring pushing it up to create a small air chamber insider the filter, thus making breathing easier.

The filter used had a male thread at the top to connect to the mask and a female one at the bottom to connect to a T.35 filter, in case there was a chemical attack on top of the already present exposure to CO.

O.39 with CO filter and T.35 filter

CO filter and T.35 connected to a modified M.33. The CO filter was never issued with this mask, but it's some times seen depicted in manuals from that era

An M.33 with the same kit described in the previous picture, from a manual of the era

Same kit, without the T.35 filter

Markings

The filter is marked on top with the producer's logo, on the bottom with the date of production and on the side with the text (bold for the original, italics for the translation):

FILTRO AGGIUNTIVO CONTRO IL CO (additional filter against CO)

DA IMPIEGARE UNA SOLA VOLTA CONSERVARE IN LUOGO ASCIUTTO (to be used only once store in a dry place)

APRIRE IL FILTRO SOLO AL MOMENTO DELL'USO E PER UN TEMPO MASSIMO DI 1 ORA (open the filter only when it must be used and only for a maximum of 1 hour)

Extra notes on efficiency

Note that the technicians of the SCM knew that, in particularly favourable conditions (dry and cool climate), the filter could last up to 2 hours, and that, if used less than 30 minutes, it could be reused for 30 to 90 more minutes. Soldiers were notified of these possibilities and also the following:

• If the CO present in the air would be more than 0,5 %, the filter would become useless
• In case of a chemical attack, it would have to be used conjoinedly with a T.35 filter, as it is not capable of filtering anything different from CO