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A local study first of its kind conducted by and Dr Sara Camilleri from the Department of Chemistry and from the Faculty of Science respectively at the University of Malta, has just been internationally published in the journal Building and Environment on The study is entitled “Impact of daily household activities on indoor PM_2.5 and Black Carbon concentrations in Malta”  

Why study the local indoor environment? 

Research in the last decade has shown when people spend about 90% of their time indoors, the overall exposure to particulate matter (PM) and other pollutants is often greater indoors than outdoors. 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. Various activities in the indoor environment also determine and effect the indoor pollutant levels.

Thousands of studies have been carried out worldwide to know which pollutants are present in the outdoor air and from where they originate. PM is known to be one of the major, leading pollutants and there is no-safe level of exposure to it. Several short- and long-term acute health effects have been well documented. Exposure to Black Carbon, also known as soot has also been linked with cardiopulmonary hospital admissions and cardiovascular mortality by the World Health Organisation (WHO). Additionally, even low-level exposure to soot has been linked to systemic inflammatory effects especially for vulnerable groups.

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

Most probably outdoor PM has more varied sources, but the indoor environment can equally contribute to substantial levels, due to various household activities. The exposure to these two pollutants, due to indoor activities has been less studied.

This Study

Typical three-storey houses represent the majority of dwellings in Malta and in the Northern district there is 18.4% of the total dwelling stock. For this purpose a house in Naxxar, surrounded by three major roads that feed traffic into the busiest Northern Harbour district had concurrent indoor and outdoor sampling of PM_2.5 (fine PM) and soot. Apart from studying pollutant levels, the rationale of this study was also to monitor the daily PM and soot levels whilst daily activities were carried out in the house, under different ventilation conditions in the winter months (November 2020-February 2021). The scope of looking into the winter months was to evaluate the scenario of a more sealed residence, with more activities going on in reduced ventilation regimes.

The important findings were:

1) Indoor PM₁₀ and PM_2.5 levels (inclusive of daily indoor activities) exhibited a larger range compared to outdoor concentrations.

2) Between 7am and 8am, a mean PM₁₀ outdoor level of 100 μgm^-3 was recorded, slightly higher than what was observed at the Msida, ERA station (80 μgm^-3), during the same monitoring period.

3) The largest increase in the fine PMlevels was recorded following sweeping of the floor followed by cooking activities such as browning of toast. The other activities resulting in an increase of this PM fraction was grilling food, lighting of candles and incense sticks (in that order).

4) The outdoor soot median level was 0.80 μg m^-3 as compared to the median indoor level of 1.01 μg m^-3.

5) The daily outdoor soot levels varied exactly as the traffic profile, exhibiting morning and evening rush hour peaks. A less of a distinctive pattern was noted for indoor soot levels. The indoor soot levels remained mostly stable throughout the day.

6) In contrast, of all indoor activities, the major contributor to indoor soot was burning of paraffin wax candles and incense sticks which were much higher than all the other activities associated with combustion.

7) As expected, high air exchange rate (ventilation) helps to reduce indoor levels of fine PM and soot.

These results are not easy to compare with other studies due to the characteristics of our buildings primarily because in our residences we do not used a filtered, centralised heating and ventilation system as is common in the US. Most of the published indoor studies generally have such systems installed. The practice is less common in other areas of the world. Also the use of tiled rather than carpeted floors facilitates wet dusting, and hence removal of some pollutants when they deposit. However, proximity to traffic of most of our urban areas contributes to some degree of infiltration of pollutants associated with tailpipe emissions and the finer portion of resuspended dust.

The latest WHO Air Quality Guidelines (AQGs) of 2021, have been updated and are more stringent. A PM_2.5 daily limit value of 15 μg m^-3 instead of the older 25 μg m^-3 is advocated. No AQGs are established for soot, so no comparisons can be made for this pollutant. The daily PM_2.5 limit of 25 μg m^-3 was exceeded on 6 and 3 days out of the 90 days of measurements in our outdoor and indoor measurements, respectively. If a 15 μg m^-3 daily limit is implemented, we find 17 and 10 days with higher daily outdoor and indoor PM_2.5 concentrations, respectively. The change in the AQGs already indicates that it is going to be a challenging feat to continue reducing further the PM levels in both indoor and outdoor environments. 

Limitations and Further Questions

The study’s main limitations were linked to the fact that only one house was tested in one town of Malta. Can the findings be generalised for the rest of the Maltese Islands? Most probably from the aspect of the type of indoor activities and ventilation practices yes. As for the contribution of the outdoor air to the indoor environment and if that significantly changes in the denser areas of our urban conglomeration or other areas having different characteristics is yet to see.

Accompanying possible difference expected due to different locations, the most probable, obvious question is: to what levels would pollutants build up in more compact dwellings/apartments, where most probably ventilation conditions are substantially different compared to terraced houses?

The most interesting scientific issue to address locally is, what is the difference in the chemical composition of the indoor and outdoor PM, keeping an eye on organic, carcinogenic pollutants associated with combustion processes, but not only? Stay tuned for a follow-up publication supplementary to this study.