Sweeping the floor and toasting bread are everyday actions many of us do at home without realising that these tasks emit fine particles into the air we breathe, a new study has found.

The study on emissions inside Maltese homes also found that actions such as burning of candles and incense sticks were the largest contributors to the generation of indoor black carbon, or soot.

Titled ‘Impact of Daily Household Activities on Indoor PM2.5 and Black Carbon Concentrations in Malta’, this was the first local study into particulate matter and black carbon emissions within local homes.

It was conducted by atmospheric chemist Noel Aquilina, from the University of Malta’s Department of Chemistry and Sara Camilleri, from the Faculty of Science.

For the purposes of the study, which will be published in the journal Building and Environment, researchers took minute-by-minute readings of emissions from outside and inside a house located in central Naxxar between November 2020 and February 2021.

Outdoor readings were taken from the roof of the house.

The indoor readings were taken from the living room of the three-storey house during various activities, some of which took place in nearby rooms. Activities involved cooking in the kitchen, sweeping the living room and lighting candles and incense sticks in the same room.

 Why particulate matter and soot?

Exposure to particulate matter (PM) is well documented to have several health impacts either in the short or long-term.

In fact, there is no safe level of exposure to PM, Aquilina says.

The toxicity and health effects vary according to from which sources the PM derives and that is why it is regulated by the EC Air Quality Directive and the new WHO Air Quality Guidelines. The latter recently lowered the daily level to 15 µg/m3 (micrograms of pollutant per cubic metre of ambient air) from 25 µg/m3.

During the 92-day measurements, this daily limit value was exceeded on 17 days outdoors and on 10 days indoors.

Soot generally comes from traffic or industrial combustion.

“In Malta, its source is the former, clearly visible in the outdoor daily profile we obtained,” Aquilina notes.

“Soot is also known to be toxic but studies are limited and there is no regulation yet. The information on levels of soot due to any combustion indoors is even more scarce, hence the study’s contribution to knowledge advancement.”

The researchers focused on measuring the presence of fine particulate matter and black carbon, inhalation of which is linked to health problems.

Particulate matter ranges in size and can be very problematic because the finer the particles, the deeper they penetrate into the respiratory tract, ending up in lungs.

Coarse particles are the relatively large airborne particles mainly produced by the mechanical break-up of even larger solid particles. They have an aerodynamic diameter ranging from 2.5 to 10 microns, about one-seventh the thickness of a human hair.

This distinguishes them from the smaller airborne particulate matter referred to as fine (PM2.5).

PM is known to be one of the major leading pollutants.

Several short and long-term acute health effects have been well documented. Exposure to black carbon has also been linked with cardiopulmonary hospital admissions and cardiovascular mortality by the World Health Organisation.

The prevalent outdoor sources of soot are either traffic exhaust or industrial combustion. In the indoor environment, soot is associated with incomplete combustion of fossil fuels, in stoves, lit candles and charring food during cooking.

It is a well-known fact that people spend about 90 per cent of their time indoors.

The study explains that the overall exposure to particulate matter and other pollutants is often greater indoors than outdoors, where there is more ventilation.

“The chemistry occurring in indoor environments is different and largely unknown because of unique sources and complex factors that are not present in the outdoor environment such as interactions of pollutants between building and furnishing materials, ventilation conditions, light, temperature and humidity,” the study says.

Worst offenders

The study found that indoor particulate matter levels exhibited a larger range compared to outdoor concentrations during certain activities.

Whilst outdoor levels of PM10 (big particles) during the measurement campaign reached a mean of 100 μg/m3 (micrograms of pollutant per cubic metre of ambient air) during the traffic rush hours, peaks of indoor levels were noticed from 6am till noon and from 1 to 6pm ranging from 25 to 30 μg/m3.

PM10 almost exclusively enter the building on opening windows and doors.

Indoor generation is more associated with fine particles (PM2.5).

The largest increase in PM2.5 concentrations was recorded following sweeping of the floor followed by cooking activities such as browning of toast. The other activities resulting in an increase in fine PM were grilling, lighting of candles and incense sticks, in that order.

In contrast to all indoor activities, the major contributor to indoor soot was burning of candles and incense sticks,  which were much higher than all the other activities containing combustion.

The researchers noted that ventilation helped to reduce indoor levels of fine PM and soot.

“The most probable, obvious question is to what extent would pollutant levels build up in more compact dwellings where, most probably, ventilation conditions are substantially different compared to terraced houses,” the researchers said.


Noel AquilinaNoel Aquilina

Noel Aquilina will be speaking live in a public talk today entitled ‘Pollution: In and Out’, organised by Malta Café Scientifique at Spazju Kreattiv, St James Cavalier at 7pm.


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