In a standard light fixture, the bulb, contacts, wiring, and any switches are all directly exposed to the local atmosphere. The spark from a loose contact or the movement of the switch, and even the heat of the bulb can be enough to ignite a flammable atmosphere. In an explosion proof light fixture, any explosions that may occur are contained inside the fixture.
Components in a standard lighting fixture such as the bulb, wiring, contacts and switches are exposed to the atmosphere. As such, any sparks caused by the sudden change in energy inside the fixture may potentially be able to ignite the flammable gases in the atmosphere. Conversely, the components of an explosion proof lighting fixture are encased to prevent potential sparks or flames from escaping the internal housing unit.
However, this does not mean that there has to be an airtight seal around the components of an ex proof lighting fixture to prevent the flammable gases in the surrounding atmosphere from penetrating the housing unit. It just means that the explosion proof lighting fixture will be able to contain the explosion within its housing unit. This prevents the explosion from spreading outside the unit which could cause additional damage to equipment and injury to employees.