As part of the PhD project of Mr Renato Camilleri, within the Department of Chemistry, supervised by Dr Noel Aquilina and co-supervised by Professor Alfred J. Vella, the findings of the first source apportionment study of indoor PM2.5 in Malta (PM2.5 is particulate matter of size smaller than 2.5 µm diameter and more commonly termed as fine dust), have been published in the international journal Atmospheric Environment.
Understanding the chemical composition of outdoor fine dust has been researched across the world in thousands of studies and the association of ambient air pollution to increasing health issues and mortality is well established.
As people spend around 90% of their time indoors, the exposure to fine dust and chemicals associated with it is important to be characterized and only recently more focus is being placed on the indoor environment which typically exhibits more complex chemistry. It is becoming increasingly important to identify the indoor sources of PM2.5 and understand the influence of the outdoor air on the indoor environment, rather than relying on the outdoor air pollution information alone. If the exposure is linked solely to the outdoor pollutant levels, one runs the risk that the personal exposure to the fine dust, based on our daily activities indoors, is underestimated.
In this study, airborne fine dust indoor samples were collected three times a week from June 2018 till July 2019 in a residence located in Birżebbuġa. This area is defined as urban background because its pollution levels are not significantly determined by a single source. A wide range of equipment available from the Air Quality Laboratory of the Faculty of Science was used for this extensive sampling campaign.
The samples were extracted at the Department of Chemistry and analysed for 18 metals at the University of Birmingham, UK, for elemental and organic carbon at the University of Wisconsin-Madison, USA and for 5 ions at the University of New Hampshire, USA. A source apportionment modelling approach, using positive matrix factorization was applied to determine the sources that do contribute to indoor fine dust, in the abovementioned site.
The mean annual concentration of outdoor PM2.5 at the site was 10.8 μgm-3, very similar to what is observed at the Żejtun, ERA urban-background air quality monitoring site (11.8 μgm-3, annual average for 2008-2017). On the other hand the mean annual concentration of indoor PM2.5 at the site was 5.7 μgm-3. Although the level is about half of what was recorded outdoors, what was it composed of? How different chemical analyses lead to the understanding of the sources contributing to this indoor level of fine dust? Do the sources change across the seasons?
Eight sources were identified for the indoor fine dust at the site, three natural and five due to anthropogenic activities. Saharan dust, sea salt and ammonium sulfate, of natural origin, together contributed to 42% of the fine dust. Saharan dust is prominent in spring and autumn and sea salt is more recorded during winter. Although ammonium sulfate can be attributed to fertilizer use, the levels obtained at the site are more linked to long-range transport from continental Europe. Mount Etna is an active volcano emitting 1.8 kt day-1 of sulfur dioxide in the atmosphere, generating ammonium sulfate as a secondary pollutant. This contribution to fine dust varies from 15% in winter to 43% in summer. The atmospheric chemistry leads to higher concentrations in summer.
The indoor sources contributed to 26% of the indoor dust. There is a wide range of indoor sources, however it is not unexpected that cooking was the activity which contributed most. As no cigarette smoking occurred in this residence, levels of fine dust generated indoors were not higher than normal.
An expected source was traffic, associated with elemental carbon and a variety of metals being found in the fine dust, coming both from tailpipe and non-exhaust emissions such as brake, tyre and road wear. This source contributed to 10% of the fine dust which infiltrates indoors and seems constant throughout the seasons. Another 10% of the fine dust which makes its way indoors comes from shipping emissions, represented by the easily detected Nickel and Vanadium, typical of heavy fuel oils used in this industry. This contribution is highest in summer.
Toxic Lead and Cadmium were also found in the indoor dust, attributable to industrial emissions (2%) from a nearby industrial zone. The last but not least contributor to indoor PM2.5 showing a statistically significant seasonality is fireworks. In summer this contribution amounts to 6% of the indoor fine dust while it is almost negligible in winter.
How do these results compare to other indoor international studies? This type of modeling was carried out in a very limited amount of indoor studies. Our indoor contribution is one of the lowest, probably because we tend to ventilate our houses adequately, yet cooking is always a major source. Traffic contributed less than other studies because of the urban fabric studied. It is well known that this contribution could easily be around three times as much (27%) in the most trafficked, urban areas in Malta as shown in a previous study. Ammonium sulfate is a phenomenon noted also in other Mediterranean countries, with several countries recording similar levels. The identification of fireworks as a source to long term indoor PM2.5 and the increase of the load of fireworks’ metals in summer to up to 6% of the gravimetric concentration is a scenario unique to our country. No international publications to date have reported this phenomenon.
In conclusion, 68% of the indoor fine dust is of outdoor origin. In summer, this contribution is higher (83%) mainly associated with long range transport of ammonium sulfate, traffic, shipping and fireworks being the most important contributors. In winter the outdoor contribution reduces to 51% mainly because there is less infiltration, as homes have windows and doors closed for longer times. In 2021, the World Health Organisation issued an updated annual air quality guideline level of 5 μgm-3 for PM2.5, applicable to both indoor and outdoor environments in order to reduce the health effects due to exposure of fine dust. Given our local, typical indoor levels, ranging from 6-8 μgm-3, this will be quite a challenging target to achieve.
And what about the health effects of the short/long-term exposure to a vast range of toxic, mutagenic, carcinogenic and genotoxic chemicals found in the fine, coarse and settled dust? If the country wants to take onboard a preventive rather than a curative approach, that would need a nationally funded programme to address the complexities associated with different exposure pathways and short/long-term health effects.