If there are dust or debris found on the surfaces of the markers, the dust or debris particles will reduce the ability of the markers to absorb and recharge the light energy, and therefore reducing the amount of light charge that are stored in the markers.  This in terms reduces the brightness output during emergency illumination, and also the duration of illumination which are both dependant on the amoung of light charge stored on the markers when the power failure occurs.

 

This is similar to the workings of a backup battery of an emergency lighting system.  If the backup battery is adequately charged and inspected and replaced when necessary, then it will provide the full duration of illumination of 90 minutes or longer as required by code.  However, if the battery is not charged properly or has not been replaced when required, then the it may only last for as little as 10 minutes during a power failure.

 

Photoluminescent systems are more vulnerable in the sense that not only the time duration is affected, but the initial afterglow brightness is affected also.  When a power failure occurs, a relatively dirty stair step marker may only provide as little as 25% brightness level of the full illumination, and may only last for 10 minutes or shorter,  as the marker was unable to reach full light charge due to the accumulated dust layer on the marker surface.