Six people have needed medical treatment after being exposed to potentially acidic dust at a West Australian lithium processing facility.

The incident occurred on Thursday at the Albemarle lithium hydroxide plant near Bunbury which is subject to an ongoing WorkSafe investigation over other workplace-related issues. 

The exposure resulted from dust from an incorrect chemical mix escaping through a vent at the site 150 kilometres south of Perth.

The Australian Manufacturing Workers Union (AMWU) said the dust was then carried by the wind, causing eye and respiratory irritation to six workers. 

One worker was left with a rash on his face. 

The facility is majority owned by US chemical company Albemarle.

"Six contractors visited the site medical centre for first-aid treatment prior to being released," the company said in a statement. 

"An internal review was conducted and further controls and mitigations are in place for any future occurrence.

"The dust presents no harm to the environment or community."

Haphazard safety approach: union

AMWU WA state secretary Steve McCartney urged the facility to take a more proactive approach to safety. 

"It's a bit haphazard ... this company has had too many issues in too short a time for us to say that they're taking [safety] seriously," Mr McCartney said. 

"We're trying to give the company a wake-up call around making sure they provide a safe workplace in the future and just think this stuff through before they worry about profit."

Mr McCartney said if workers had not previously raised concerns about a lack of working chemical eye-rinse stations, things could have been worse. 

"If those guys didn't have the courage to stand up in the first place, then people would be wondering where they can wash their eyes out." 

Facility already put on notice

The Department of Mining, Industry Regulation and Safety (DMIRS) launched a WorkSafe investigation into the Albemarle site in early April after being alerted to multiple safety concerns.

An incident saw a number of workers hospitalised as a precaution after being exposed to toxic gases.   

The facility is being both constructed and operated simultaneously, meaning a mix of construction and chemical workers are present on site.

Last week, WorkSafe issued Albemarle with an improvement notice that compelled the company to install safety barriers between the operating and construction sections. 

Albemarle has until July 15 to comply with the notice. 

The WorkSafe investigation is ongoing, with the department still assessing other safety issues.