As smoke from out-of-control bushfires has blanketed Australia's Sydney and triggered health warnings from authorities for much of the past month, experts warn that face masks may not offer as much protection as some people believe.

Head of the environmental health group at the Menzies Institute for Medical Research, Associate Professor Fay Johnston, told the Sydney Morning Herald on Friday that most commercially available masks do not filter the finer particles in bushfire smoke.

"That really won't protect you from the health effects of smoke because it's the finer particles that we worry about with smoke, so they're not a good solution all in all," Johnston said.

Studies have suggested that those using paper or cloth masks are likely receiving no benefit, and the only masks likely to work in the smoky conditions would be a P2 mask, worn by builders and available at hardware stores.

Over the past two months Sydney has experienced more hazardous air quality warnings than over the previous five fire seasons combined.

While health authorities have advised, young people, the elderly, and those with respiratory issues to stay indoors, the longer the smog lingers, the more locals are raising concerns regarding health effects.

Bushfire smog contains larger particles as well. However, it is the smaller ones known as particulate matter (PM) 2.5, which are so small that they can penetrate into the bloodstream.

"These fine particles can penetrate deeply into our lungs and they can also cross into the blood system and affect other organs in our body," Christine Cowie, an expert at the Centre for Air Pollution, Energy and Health Research, told Xinhua earlier in the week.

"There's really strong evidence for the effects on the cardiovascular system and (over prolonged periods) those effects manifest themselves in an increased risk of heart attacks and stroke primarily."