Fires are burning across New South Wales, and while the main fires causing smoke in the ACT are the Currowan and Braidwood fires to the east, there are also fires to the south-east, north-west and south-west.

Health professionals are advising people with existing respiratory issues like asthma to stay inside and avoid strenuous activity when the air quality deteriorates, but as smoke rolls in and out of the capital it is hard to know when is going to be the best time of the day for necessary outings.

Some of us are now willing to walk the dog or go for a run in a level of smoke we would previously have considered bad, but now know isn't the worst we could experience.

We may want to leave the windows open to encourage fresh air but close them before a new front of smoke arrives.

So how do we know what's coming?

We don't, and predicting levels of smoke is more fickle than predicting the weather, says acting chief health officer Paul Dugdale.

"Forecasting smoke is quite difficult, because you've got not just the weather, not just where the fires are, but what the activity is in the fires and whether it's burning smoky or less," he said.

So it's not as simple as seeing an easterly wind forecast and assuming it will come with smoke?

"I would tend to rely on common sense and have a look at the air and make your decision based on that," Dr Dugdale said.

Dr Dugdale advises people make decisions based on the air as they are experiencing it, and also their knowledge of their own bodies.

"Just have a look around. If it's got smoky, then then take it easy and get back inside," he said.

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Dr Dugdale said everyone is different and their own best judge of when they should stay inside or choose the gym over a run outside.

"For people generally it's an aesthetic consideration as much as a health consideration and I don't think people should worry about when to go to the shops or when to go and visit some friends, but on the other hand the height of summer is never a great time to plan an outside picnic if it's going to be 39 degrees like it is today."

Meteorologist Helen Reid from the Bureau of Meteorology said the smoke could be worse on New Year's Day.

"I would expect smoke haze to get worse on Wednesday with that cold front moving through on Tuesday. When that has gone through and crossed over the south east of the country, it will bring an easterly flow ... that will bring in smoke from fires that are to the east of the ACT," Ms Reid said.

Dr Dugdale said most locals will have noticed the smoke tends to increase in the late afternoon and evenings, so some planning can be made around that.

"Plan to be out during the cool of the morning when the air quality is usually better."

"It's not particularly dangerous for the sorts of period of time that we've had, you know, this is not a year-round smog that some cities have to contend with," he said.

"This is a bad summer of bushfires and we'll get through it."