What is Mrs. Green?

Mrs. Green was the manager of a large company, and frequently had to have meetings with other business people in a room in her building. She did not smoke at all, but many of the other people at the meetings did so. She often found the air during the meetings terrible. One day, after an hour, her throat and eyes were sore, she was coughing a lot, so she called a big air-conditioning company and asked them to work out how much it would cost to keep the air of the meeting room in her building really clean.
   After a few days the air-conditioning company sent two estimates for Mrs. Green to choose from. One estimate was $5, 000 to put in new air-conditioning, and the other was for $5. 00 for a sign which said, "NO SMOKING".