Authorities are warning NSW residents not to underestimate the damaging effects of smoke as air quality worsens amid the state's bushfire crisis.
About 80 fires are burning across NSW, filling Sydney's skies with smoke pollution of "hazardous" proportions, according to the Department of Environment.
The latest air quality index rating has recorded 221 micrograms of particulates per cubic metre in Sydney's east, while the lower Hunter's figure of 306 was the worst in the state.
Dust has also contributed to poor air quality in Sydney, Newcastle and the Central Coast ahead of a southern change expected to hit on Tuesday evening, the Bureau of Meteorology said.
NSW Ambulance commissioner Dominic Morgan said the smoke was causing "slightly above-average demand" on services, with almost 100 asthma or breathing related calls since Tuesday morning.
Of those, 39 were from the Sydney area and another 35 were from the north of the state where the majority of fires are burning.
In one case paramedics treated a patient who no longer had their reliever medication.
Mr Morgan 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," he said.
"So make a plan and get medical aid as soon as possible."
Australian National University population and climate health expert Liz Hanna said everybody should be wary of the 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, Dr Hanna said.
"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 authorities for people to remain indoors and avoid outdoor activity, noting long-term exposure to pollution can lead to problems later in life.
"You can have scarring in which case your long-term respiratory function is diminished," she said.
"All this stuff can sensitise them and give them lifelong problems with breathing and greater susceptibility to colds and coughs and shortness of breath when confronted with other problems."
Australian Associated Press