Waking up to a city full of smoke. It's the new norm in Sydney.

The spate of bushfires burning across NSW has given people in the Harbour City a taste of what it's like to live in a smog-choked megacity.

NSW Department of Environment data shows Sydney's air quality this bushfire season is already three times worse than at any moment in the past five years.

This graph shows the amount of harmful pollutants in Greater Sydney's air since the start of this season's bushfire crisis compared to previous years.

The AQI in Greater Sydney has been particularly hazardous this bushfire season. ( ABC News )

The Department uses an Air Quality Index (AQI) to inform the public about pollution.

The AQI is calculated by measuring the amount of "particulate matter", or microscopic pollutants, in the atmosphere.

Key points: The air quality in parts of Sydney plummeted in November

The air quality in parts of Sydney plummeted in November In some areas, it went from the equivalent of smoking half a cigarette each day to almost 10

In some areas, it went from the equivalent of smoking half a cigarette each day to almost 10 But experts say little is known about the long-term health effects of prolonged exposure to bushfire smoke

While NSW Government measures several different categories of "particulate matter", the one of most concern to people's health is PM2.5.

It's also the smallest size the NSW Government measures — about 3 per cent of the diameter of a human hair.

That means it can enter a person's lungs and bloodstream, and can trigger heart attacks, strokes, lung cancer and asthma attacks.

In the past five years, albeit when there were fewer monitoring sites, there were only five recorded instances of a daily maximum AQI above 100, indicating "poor" air quality in Greater Sydney.

In November and December this year, there have already been about 80 — over a quarter of these readings were above 200, which indicates "hazardous" air quality.

In health terms, the toxicity of the air in some parts of Sydney has gone from smoking half a cigarette, to between four and 10 per day.

On Tuesday, the AQI hit 669 — the equivalent of smoking 30 cigarettes.

Bushfire smoke cast the skies of Sydney in a hazy glow on Tuesday. ( Supplied: Samantha Skinner )

Director of Environmental Health at NSW Health, Dr Richard Broome, said there had been an increase in the number of people presenting to emergency departments with respiratory issues.

During the week ending November 22, there were around 2,230 ambulance calls for breathing problems — a 22.5 per cent increase from Sydney's weekly average.

In the Mid-North Coast, where bushfires were so intense they created their own weather systems, the number of ambulance calls for breathing problems doubled over the same period.

Christine Cowie, a senior fellow at the Centre for Air Pollution, Energy and Health Research at the University of NSW said breathing in the high amount of fine particles could affect physical development.

"It is uncertain how medium-term exposure to these sporadic bushfire pollutions events impact on long-term health," she said.

"However, we do know that current evidence indicates there is no safe lower threshold of exposure to [particulate matter] pollution."

Dr Cowie said while some studies were underway, there was limited research looking into when a city is blanketed in smoke for weeks like Sydney.

"It is difficult to to definitively say what happens, so in the meantime we recommend taking the normal health precautions," she said.

"Certainly people who are repeatedly exposed to high levels [of air pollutants] and if they're children for instance or elderly, it's likely to impact on their lung function."

"No one can say definitively what happens after a two-month exposure to those high levels, other than if you're susceptible, you're likely to have increased respiratory problems," she said.

Parts of Sydney shrouded in smoke on Tuesday night. ( ABC News: Sarah Gerathy )

A recent study, published in the British Medical Journal, found that short-term exposure to fine particles of pollution is linked to hospital admissions for a wider range of medical conditions than previously thought.

Alongside lung cancer and heart attacks, the researchers found air pollution was also connected to septicaemia, UTIs and infection.

This unexpected finding could be explained by the inflammation the tiny particles cause in the body, the study's senior author Professor Francesca Dominici told ABC's Health Report.

"Even though we don't know yet all of the clinical pathways that could have led to this disease [lung disease], we do know that inhalation of fine particulate matter penetrates deep into the lung and initiates a series of inflammations that could simultaneously affect multiple organs."