As Sydney's air pollution hits hazardous levels amid raging bushfires, authorities are warning asthmatics and others to remain wary of the smoke.

Authorities are warning NSW residents not to underestimate the damaging effects of smoke as air quality worsens amid the state's bushfire crisis.

More than 78 fires are burning across NSW, with 47 uncontained and 12 elevated to "emergency" warning level.

The blazes have lowered air quality, with all regions in Sydney now deemed hazardous because of the smoke haze.

The Department of Environment's air pollution index rating has recorded 239 micrograms of particulates per cubic metre in Sydney's East, while the Lower Hunter's figure of 340 was the worst in the state.

NSW Ambulance Commissioner Dominic Morgan said the hazardous air quality had resulted in "slightly above average demand", with 31 calls for respiratory conditions since 5am including 17 in Sydney.

One case was related to a patient who no longer had their reliever medication, Mr Morgan said.

He urged asthmatics and others vulnerable to respiratory issues to remain vigilant, or the consequences could be fatal.

"To fail to make a plan when you have an asthma condition or respiratory condition and not have reliever medication, this could be a deadly condition for you," Mr Morgan said.

"So make a plan and get medical aid as soon as possible."

But Australian National University population and climate health expert Liz Hanna insisted everybody should be on notice to smoke's irritating effects.

"Never underestimate how these things can sneak up on you and how damaging they can be, even if you think you're fit and well," Dr Hanna told AAP.

"Smoke can amplify pre-existing conditions but also trigger responses in people that don't know that they're sub-clinical.

"They can have quite a dramatic response and this could be their first time.

"That's when it's really quite dangerous because they don't know they're vulnerable and they tend to not have the medication by their side."

Inhaled heat can also burn people's throat and lungs, with Dr Hanna stressing the need for citizen firefighters to wear proper attire.

"There's a wicked tendency for people to get out there in their thongs and shorts," she said.

"That offers zero protection to the radiant heat of a fire.

"They should wear thick cotton fabric rather than some of the artificial fabrics that can actually melt and burn the skin."

Dr Hanna echoed the advice of NSW Health for people to remain indoors and avoid outdoor activity.

Long-term exposure to air pollution can lead to problems later in life, she said.

"You can have scarring in which case your long-term respiratory function is diminished," Dr Hanna said.

"All this stuff can sensitise them and give them life-long problems with breathing and greater susceptibility to colds and coughs and shortness of breath when confronted with other problems."