By some measures, Canberra has been the world's most polluted city for days, surpassing even notoriously polluted metropolises including Delhi, Karachi and Beijing.

According to Health ACT, as at 11am on Monday the air quality index, which measures the concentration of fine particles in the air smaller than 2.5 microns, was between 320 and 516 across the city, a vast improvement from earlier in the day when levels reached above 2500, but still above the 250 point mark which indicates extremely hazardous pollution.

According to canberraair.com, which uses hourly air quality readings, pollution levels were even lower, between 79 and 110 by early afternoon on Monday.

The air quality index exceeded 4400 points on Sunday, and reached 4650 on New Year's Day.

Why has the index stayed so high even though conditions are clearing?

Official readings from ACT Health are based on a 24-hour rolling average of readings from three monitoring stations, in Florey, Civic and Monash. This is the measure prescribed under the National Environment Protection Measure agreed to by the states and territories.

Also, because Health ACT uses a 24-hour rolling average, its index will move more slowly than changes in hourly pollution readings, such as those published by canberraair.com or the AirRater app.

Why do readings differ so much between sources?

The air quality index is arrived at by applying a multiple to actual particle readings.

Because this multiple is not standardised, organisations can arrive at a different index based on the same actual reading.

Why has the smoke been hanging around Canberra?

For most of the past month Canberra has sat in the path of smoke coming from massive bushfires on the South Coast. While westerly winds during the day have tended to bring clearer air, in the evenings on shore winds have prevailed, blowing smoke from the coastal firegrounds west toward and over Canberra.

This situation has been exacerbated in recent days as large fires to Canberra's south and west have developed, increasing the amount of smoke blowing across the capital from multiple directions.

How bad is the bushfire smoke for my health?

Health studies have found a significant association between particle pollution and health problems including asthma attacks, bronchitis, heart attack and cardiac arrhythmia.

Fine particles of less than 2.5 microns are thought to pose the greatest health risk because the can lodge deep in the lungs and even enter the bloodstream, impairing breathing and potentially damaging tissues.

In a paper on the health effects of climate change, the Australian Medical Association warned in 2015 that "increased frequency and intensity of bushfires, drought and dust storms, with corresponding acute increases in [particulate matter] is ... likely to lead to increased asthma exacerbations, respiratory medication use and hospital admissions for asthma and other respiratory conditions".

Scientific studies have found even short-term exposure is associated with increased rates of hospitalisation, and prolonged exposure is associated with serious health problems. The World Health Organisation attributes millions of deaths each year to the effects of ambient air pollution.

Health ACT warns lengthy exposure can aggravate existing lung conditions such as chronic bronchitis, emphysema and asthma, and can worsen chronic heart problems.Bigger smoke particles, between 2.5 and 10 microns, can irritate eyes, nose, mouth and breathing passages, but the body is better able to filter these larger particles.

Who is most at risk?

People with respiratory conditions, children, the elderly and pregnant women are considered to be most vulnerable to the health effects of smoke.

What can I do to lessen my risk?

Health ACT has advised people to stay indoors where possible, and to avoid using evaporative air coolers, which draw on outside air.

The department says that ordinary paper face masks are not effective at filtering smoke, but do not cause any harm.

Evidence shows that P2 and N95 masks do filter some smoke, but because they are difficult to fit and use effectively Health ACT says such masks cannot completely eliminate smoke exposure and staying indoors is the best option to protect your health.

Air purifiers fitted with HEPA filters can remove PM2.5 particles from the air, but need to matched to room size to be most effective.

Does air conditioning make the air safe?

Even if your building is fitted with ducted or reverse-cycle, as opposed to evaporative, air conditioning, that does not mean the air is safe.

While air conditioning systems may filter out larger particles, unless they are fitted with the right kind of filter they will not remove fine PM2.5 particles from the air.