How to check for positive or negative air flow around doors with a Smoke Pencil?

There are many hospitals that use the Smoke Pencil to check for positive or negative airflow around doors and ventilation registers to ensure that certain rooms in the hospital are not leaking air into other rooms.

Currently we are in the midst of a Corona Virus (COVID-19) outbreak in the United States. Hospitals are working feverishly to ensure their clean rooms, operating rooms, labs, quarantine rooms, waiting rooms, etc… are all separated effectively. This means the air in one room is not exchanging with the air in adjoining rooms.

Hospital maintenance staff often use the Smoke Pencil to check the airflow around door seals and vents to make sure the airflow is going in the correct direction and ventilation is operating properly. One advantage to using the Smoke Pencil is its vapor is a simple food grade mixture of Glycerin, Propylene Glycol, and water. This is much cleaner than using a smoke emitter to check airflow.