Workers including ticket inspectors, airport ground staff and council gardeners could walk off the job on Friday under a trade union directive to stop work if the state's environment agency deems the air from bushfire smoke "very poor".

Outdoor work has continued in many workplaces this summer while smoke from the fires in East Gippsland and the state's north-east hovered over Melbourne and other parts of the state.

Smoke has blanketed Melbourne at times this summer. Credit:Joe Armao

People have been urged to stay indoors as unhealthy air has persisted for days. Hardware stores have run out of masks and smoke alarms have been triggered inside CBD buildings.

The trade union movement has reacted to the new workplace health and safety problem by developing a joint policy to ensure workers' health is not risked.