What are the regulations on the wearing of the CBRN patch?

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DA Pam 670-1, para 21-30. The CBRN patch is a "subdued brassard" as defined in para 21-30b(2).
Interestingly, you are only supposed to wear it when actively doing CBRN activities, not just for bumming around post. I just learned something today.

a. Authorization. Brassards are worn as ID to designate personnel who are required to perform a special task or to deal with the public. Brassards are authorized for wear only while actively engaged in the duty associated with the brassard and identification of personnel is required, such as field operations and event response. Brassards are not intended for wear while performing daily or routine job related activities. For example, a member of an Explosive Ordnance Disposal (EOD) Team would wear the EOD brassard while actively conducting disposal operations in an environment where non-EOD personnel are present and identification of EOD personnel is necessary; the brassard is
not worn while conducting staff activities, routine maintenance and preparations, or while in an on-call or stand-by status.
b. Description.
(1) Nonsubdued. Nonsubdued brassards are made of cloth 17 to 20 inches long and 4 inches wide of colors specified below. When more than one color is specified for the cloth brassard, the colors are of equal width and run lengthwise on the brassard. Cloth brassards are worn on the left sleeve of the outer garment, with the bottom edge of the brassard approximately 2 inches above the elbow (see fig 21–145).
(2) Subdued. Subdued brassards are a rectangular shaped embroidered device with an overall dimension of 1 13/16 inches in height by 3 1/4 inches in width, all within a 1/8 border. The subdued brassard is a hook-an-loop insignia and is worn attached to the left sleeve of the combat uniform coat shoulder pocket flap (see fig 21–145).
...
(4) Chemical, Biological, Radiological, and Nuclear brassard.
(a) Nonsubdued. The nonsubdued CBRN brassard consists of the letters “CBRN” in golden-orange block letters, on a cobalt blue background.
(b) Subdued. The subdued CBRN brassard consists of four black letters “CBRN” 1–11/32 inches in height (see fig 21–150).

This is an outstanding question!