Final answer:
The question asks to describe a NATO chemical marker, but the provided information does not include its description. Instead, details about NATO's activities, the Iron Curtain, and nerve agents are given, none of which describe the physical appearance of a NATO chemical marker.
Step-by-step explanation:
A NATO chemical marker is used to indicate areas that are contaminated with chemical agents. It's a crucial aspect in ensuring the safety of military personnel as well as civilians during and after conflict. However, the information provided does not include a description of what a NATO chemical marker looks like. Instead, we have information on NATO's activities, the history of the Cold War demarcations, and the characteristics of nerve agents.