Answer;
2810
Referring to the emergency response guidebook, the united nations identification (un id) number for the chemical agent lewsite is 2810.
Explanation;
UN identification numbers , are normally four digit numbers that identify hazardous substances and articles, and also dangerous goods. These include explosives, flammable liquids, toxic substances, in the framework of international transport.
In this case the UN id for a chemical agent called Lewisite is 2810.