The vesicant pen is an ingeniously nasty device that may be found on these pages. It may be encountered in Transhuman Space (THS) or other scenarios.
The vesicant pen looks like a large felt-tip pen or paint dauber. It contains a quantity of blister agent. For obvious reasons, the cap is airtight and this may be a recognition feature for those that are aware such devices exist. The blister agent used is clear and transparent but somewhat oily. It may stain absorbent materials or interact with some woods, plastics or varnishes.
In game terms the blister agent is treated as mustard gas (p.B439). The actual formula may be different to actual early 20th century mustard gas. For example, the updated blister agent remains liquid and volatile down to 14 °F (-10°C) and includes necrotic enzymes. THS-era blister agent is probably odourless. Characters or creatures with an abnormally sensitive sense of smell may smell something like garlic, horseradish, mustard plant or mown hay. They may not recognize this as a threat and curiosity may be tempt them to inhale more.
The simplest way to use the pen is to paint the agent on something that the intended victim is expected to touch. This can create an unpleasant but relatively localized wound. The pen may also be used to apply the agent to blades, arrows, punjis etc.
Using the blister agent for a respiratory effect is a little more difficult since the capacity of the pen is limited. Several minutes would be needed to contaminate a relatively small room. Would be users should bear in mind that blister agents are heavier than air. Blister agent is a liquid rather than a gas and its airborne form is a mist rather than a vapour.
More practical is to apply the agent to something the target will be expected to smell or have near their nose. If an individual is in the habit of sniffing marker pens, the pen can be left where they will find it. Alternately the agent can be applied to the inside of a facemask or space helmet.