Australian bushfires have produced so much smoke, US space agency NASA has aid the smoke has now completed a circuit of the Earth, reaching a height of 17.7 kilometres above sea level. NASA satellites show the smoke has crossed the Pacific Ocean, reaching parts of South America.

As a result of the smoke, the Southern NSW Local Health District says presentations to hospital emergency departments for asthma or breathing problems have been higher than normal.

In the seven days leading up to January 12, the South East Regional Hospital had 49 people present to the emergency department with respiratory related problems, compared to the hospital's five year average of 39. Cooma Hospital saw 180 presentations in total, 14 of which were respiratory related.

According to the NSW Environmental Protection Agency, bushfire smoke consists of a mixture of differently sized particles, water vapour and gases, including carbon monoxide, carbon dioxide and nitrogen oxides and many other compounds.

Larger particles visible to the eye are generally too large to be breathed deeply into the lungs but can irritate the nose and throat, while finer microscopic particles and gases can cause health issues. The agency also recommends keeping pets indoors during smoky days.

According to NASA, large areas above bushfires currently burning are currently "spewing extreme amounts of aerosols into the atmosphere creating a health hazard not only for residents in the area, but also for those affected when wind patterns carry that smoke on jet streams".

The bushfires have so far burnt more than 5.2million hectares in NSW and 1.3million hectares in Victoria, creating what NASA describe as rare pyrocumulonimbus events, where moisture trapped in smoke condensed in the cold upper air creates a cloud, which in turn creates lightning.

The NSW Department of Planning and Environment's rural air quality network is monitoring bushfire affected areas, with Wednesday's rating for Merimbula considered safe. Similar levels were seen in Batemans Bay on Wednesday, while the air in Cooma was rated as almost hazardous.

The air quality in West Gippsland, the Latrobe Valley and East Gippsland hit hazardous levels on Wednesday.

According to the Bureau of Meteorology, with rain, thunderstorms and light winds predicted for the Bega Valley region on Thursday, the smoke haze is likely to ease by Friday.

Air quality in some areas of South Eastern Australia has been so bad it has ranked as the worst in the world, with Melbourne ranking as the worlds worst city on Wednesday.

The town of Albury had an Air Quality Index of 1145 on Wednesday, almost six times the hazardous level of 200.

This week, UnionsACT secretary Alex White said there is "no safe level of exposure" to the smoke, especially the smaller particulate matter.

"The bushfire smoke that has covered Canberra since last year is toxic," he said.

Southern Director for Public Health Tracey Oakman said residents should take precautions due to the continued poor air quality, and stay indoors with the doors and windows shut to reduce continued exposure to the smoke.

Ms Oakman said residents should follow their doctor's advice about medicines and to follow asthma management plans.

"People who are at greater risk from bushfire smoke include those with heart or lung disease, such as asthma or emphysema, the elderly, young children and pregnant women," Ms Oakman said.

"Those at risk from heat stress are similar to bushfire smoke but also generally include those with a chronic disease or already sick, such as kidney disease or currently suffering from gastroenteritis; people who live alone or may be socially isolated; and those working in a hot environment.

"If you are unsure whether you are at risk, discuss it with your GP," she said.

Residents with rainwater tanks should also be aware of specific advice around their use following a bushfire, she said.

"Bushfires generate large amounts of smoke and ash, and your tank water could have become contaminated from debris and ash or dead animals, Ms Oakman said.

"If the water tastes, looks or smells unusual do not drink it or give it to animals."

A health district spokesperson said the best way to reduce exposure to smoke is to stay indoors, and to open doors and windows in clear periods to air out any smoke that may have leaked in.