You might start by detailing how the product is made

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and then how far back to take things ?

include the transport from raw material to processing to finishing ?
include mining (harvesting?) raw material,
include processing, finishing, installation (like cutting the stock panel to suit) ...

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

The OP said "exactly"

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so that's included the CO2 from the workers, getting to work at the various factories, the CO2 from making the tools used in processing, etc, etc.

ie, there is no "exactly" ... well, nothing quantifiable.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

I'm not the one setting impossible bars

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How much do we consider the transport and production costs of food/drink the workers consume? What about respiration rates of the works, what if one works if more fit and weighs less than another, need to factor for that as well. Lots to think about....

@ChorasDen you don't spec a workout regimen and dietary plan for site workers and fabricators in your general notes? The planet is melting, we simply can't afford that!

we're just trying to answer the question (essentially unanswerable) posed ... "how can I detail exactly CO2 emissions for the construction of a plasterboard pannel".

what activities do you account for in the production of something ? The factory producing the panel is obvious (I personally wouldn't count the employees as I figure they'd be alive without the production of the panel; if you do count the employees, do you count the food they eat, the gas they burn getting to work ? ...)

Do you include the production of the raw materials ?

Do you include energy costs ? What if the plant was run on solar ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

Quote (Luceid)

you don't spec a workout regimen and dietary plan for site workers and fabricators in your general notes?

Alas, I do not. Much to the dismay of several architects I work with, we are not that detailed with life cycle analysis.

The plasterboard pannels do not emit C02 in their installation but it can be produced in the manufacturing process. So in my opinion it would be necessary to look at the company's environmental impact certifications. Among the certificates you can look at are Certificate ISO 9001,Certificate ISO 14001, ISO 14025., that it is included in the Environmental Product Declaration (EDP).
I hope you find it useful,
Best regards.

Scope 1, Scope 2 and Scope 3 emissions are typically the terms used to refer to the different levels in terms of inputs, start by looking at those definitions to get a feel of what you need to look at.

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Why yes, I do in fact have no idea what I'm talking about