Hayley Hunt has fond memories of Sydney's vivid blue skies and pure air.

"Every time we go back home to Sydney, you can actually feel the air is much cleaner to breathe," she said.

"You can see everything crisper and sharper.

"I feel like I want to just take big gulps of air, it is palpable how clean the air looks and feels."

The expat now lives in Hong Kong where life can sometimes be dictated by air pollution — something Sydneysiders only have to seriously worry about a handful of days a year.

Sydney ex-pat Hayley Hunt says on bad pollution days time outside must be limited. ( Supplied: Hayley Hunt )

Yesterday was one of those, when a thick coat of smoke covered some areas, making air pollution levels higher in Sydney than Hong Kong, Beijing, Jakarta, New Delhi and Los Angeles.

Pollution was deemed hazardous in the east, north-west and south-west during different parts of the day, meaning everyone was advised to significantly cut back on outdoor activity.

The smoke-filled skies, the result of hazard reduction burns around Sydney, kept NSW Ambulance busy, with a huge increase in cases involving breathing difficulties.

It is not the first burn conducted by the Rural Fire Service this season, and for almost half of May air quality has been rated "poor".

Many expressed their concern about the smoke inhalation, with some taking aim at RFS on social media for "poisoning" vulnerable people, while others compared Sydney's pollution to that of New Delhi.

But it must be remembered, last year Sydney met the national daily standard for air quality for every day bar 23 — of which, hazard reduction burning was mainly responsible.

The Sydney Harbour Bridge was barely visible through thick smoke yesterday. ( ABC News )

Sydney is fortunately a long way off cities like Hong Kong, where air pollution is a perennial issue and often exceeds levels recommended by the World Health Organisation.

Ms Hunt is now in the routine of checking an air pollution app every day and describers herself as "paranoid".

She will limit the amount of time she and her two young children spend outside on bad days and said the culture is to treat bad pollution days like rainy days.

She has even invested in an air purifier for their house but said schools, who often cannot afford purifiers, sometimes shut down when air pollution reaches extreme levels.

Some people are "driven mad" obsessing over air pollution, Ms Hunt said, a feeling she became familiar with while living in Kuala Lumpur in 2015 during the Indonesian forest fires.

"We could not go outside for two months," she said.

"You could feel it in your eyes, feel it in your nose.

"It was so unbearable, I wouldn't have been outside for all the money in the world."

Fortunately, smoke over Sydney is expected to dissipate today, a welcome relief for many with asthma and respiratory conditions.