Hazmat teams attended the scene at roughly 4:30pm local time (10:30pm GMT).
An EMS task force was also despatched – a group usually sent to mass casualty incidents.
At least five people are reportedly being treated for symptoms including burning sensations and nosebleeds.
The exact details of the incident or the substance involved are currently unknown.
Roads around the area have been cordoned off as emergency services respond.
It is currently unclear if the incident was an intentional attack.
Local police said: “Our Hazmat Team members and Aide 215 are assisting the Fort Myer Fire department and Arlington Firefighters on a Hazmat Incident at the Fort Myer Base.”
More to follow…