Be Aware of the Indoor Air. Physicochemical Characterization of Airborne Fine Particles in Occupied Homes

Negative health effects of exposure to particles of outdoor origin have been confirmed by epidemiological studies. In developed countries, we spend on average 65% of our time in our homes. Thus, the properties of airborne particles indoors need to be understood. The aim of this PhD thesis was to investigate the differences in physicochemical and toxicological characteristics of fine particles (PM2.5) inside and outside occupied homes, as well as to understand the contribution of indoor sources to exposure indoors. The effects of energy renovation and of the occupants' activities on indoor concentrations were assessed.
Indoor and outdoor differences in physicochemical and toxicological characteristics of PM2.5 were studied in 15 homes in urban and rural areas of southern Sweden. PM2.5 characterization was performed with online state-of-the-art techniques, simpler portable instruments, and with offline methods. The occupants’ self-reported activities were used to identify the contribution of indoor sources and for interpretation of the results. Measurements in homes were supported by a laboratory study focused on the characterization of particle emissions from candles under stressed burning conditions. An in-vivo toxicity study in mice was performed to assess the differences in toxicological properties of PM2.5 collected indoors and outdoors by evaluating inflammation in bronchoalveolar lavage cells. To understand if the energy renovation affect particle concentrations, measurements were performed inside and outside of seven occupied apartments over three consecutive years, before renovation, after renovation and at a follow-up.
High number of ultrafine particles (UFP) were observed mainly due to the presence of indoor sources such as cooking, and candle burning in homes. Some of the indoor sources additionally contributed to elevated PM2.5 and black carbon (eBC) mass concentrations. In one apartment, a detailed online characterization using a mass spectrometric technique showed that PM1 emissions from indoor sources (e.g., cooking, e-cigarette vaping) were dominated by organic matter (86% of the total mass). Positive Matrix Factorization (PMF) source apportionment of the organic particle fraction showed that the largest contributors to indoor PM1 were e-cigarettes (50%), followed by cooking (40%), and outdoor infiltration was a minor contributor (10%). Candle burning, under stressed burn conditions in the laboratory experiments, emitted large amounts of UFP number concentration, PM2.5 mass and eBC mass concentrations. The wax and wick composition influenced emissions of eBC, PM2.5 and particle-phase polycyclic aromatic hydrocarbons (PAHs). In homes, candle burning also contributed to elevated levels of UFP, PM2.5 and eBC mass concentrations.
In 15 homes, the chemical composition of particles indoors and outdoors was different regarding metals, PAHs and endotoxins. Higher concentration of metals such as Fe, Cr, Al, Zn and Mg were found in particles collected indoors compared to outdoors. This was most probably due to cooking, candle and incense burning. Indoor particles collected in 15 homes showed higher toxicity compared to those collected outdoors. This was most likely linked to higher levels of metals, polycyclic aromatic hydrocarbons (PAHs) and endotoxins in particles collected in all homes indoors in comparison to outdoors.
After energy renovation, the UFP concentrations did not decrease and the observed concentrations were mainly affected by the occupants' activities. In order to reduce exposure to UFP particles, more stringent building regulations for kitchen extraction hoods should be considered. The indoor PM2.5 mass concentration had decreased at the follow-up. This could be a result of a lower amount of PM2.5 generated from indoor activities at the follow-up compared to before renovation, and of decreased infiltration of outdoor particles due to the renovation.
The knowledge obtained in this thesis can be used for developing appropriate strategies to minimize exposure to particles indoors. A combination of methods is needed to effectively remove particles generated indoors and to prevent outdoor infiltration. Additionally, the data presented here can be included in mapping of real-life indoor concentrations and in development of indoor air quality models for exposure assessment.

How much time do you usually spend at home? On average, it is 65% in developed countries, which is about 16 hours per day. It is known that exposure to airborne particles of outdoor origin can cause adverse health effects. As we are spending most of our time indoors, we are exposed to airborne particles of different kinds than outdoors. Thus, we have to understand where particles come from, what their properties are, and which sources contribute to indoor levels.
But what are the sources of particles in our homes? Particles can often be generated from activities like cooking, burning candles or incense, use of cleaning products, aroma and salt lamps, and smoking. These indoor sources generate particles smaller than 2.5 micrometers in size, these are called fine particles (PM2.5). Particles also infiltrate from outside though the ventilation system, cracks in the building or just through an open window or door. Additionally, new particles can be formed indoors during chemical reactions between particles and gases of both indoor and outdoor origin.
This PhD thesis focused on the characterization of different properties of fine airborne particles in occupied homes and considers contribution of both indoor and outdoor particle sources. It also investigates the influence of energy renovation and occupants’ activities on particle concentration indoors. Additionally, this thesis investigates if particles indoors differ in toxicity in comparison to outdoor particles.
We measured PM2.5 inside and outside of 15 homes in southern Sweden. We used online instruments to monitor particle concentrations in real-time and particles were also collected on filters. Occupants reported indoor activities in logbooks and we could link these activities to observed peaks in the particle concentration data. The particles collected on filters were analyzed to determine chemical composition of the PM2.5 and the particles were also used for toxicological studies in mice. During our measurements in homes, we found that candle burning was a strong source of particles. Measurements in the laboratory were hence performed to assess the particle emissions from candles under stressed burning conditions. The effects of the energy renovation and occupants’ activities on particle concentrations were assessed before, after renovation and at the follow-up.
Measurements in fifteen occupied homes showed high concentrations of very small particles called ultrafine particles (UFP, <100 nm), of PM2.5 and of soot. Indoor activities such as cooking, e-cigarette vaping, and candle burning were the major contributors to the indoor particle levels of UFPs. PM2.5 and soot were influenced by both indoor sources and infiltration of particles from outdoors.
Chemical composition of particles emitted during cooking (frying, using the oven, deep-frying) and e-cigarette vaping were dominated by organic material. Particles collected indoors had a higher concentration of metals compared to outdoors. These metals might originate from the food itself or from kitchen equipment when it was heated.
Candle burning emitted high concentration of particles. If the candle flame burns steadily, then the wax reacts with oxygen in the air and carbon dioxide, water and inorganic emissions are produced. However, in real indoor environments the candle flame is frequently disturbed by air movements (e.g., by opened door or window or by people moving around) and this will make the flame flicker resulting in incomplete combustion and hence soot formation. We found during measurements in homes and in the laboratory that candles under stressed burning conditions emit a lot of soot. The wax and wick composition is important as it influences emissions of soot, PM2.5 and particle-phase polycyclic aromatic hydrocarbons (PAHs).
The toxicological studies we did in mice showed that indoor particles collected in 15 homes were more toxic than outdoor particles collected near the homes.
We have assessed the UFP number and PM2.5 mass concentrations before, after renovation and at the follow-up. It was concluded that the energy renovation did not influence concentrations of UFPs and that these concentrations were mainly influenced by occupants' activities. PM2.5 mass concentration were lower at the follow-up. We found out that it was both due to a decrease of the indoor activities at the follow-up as well as to a reduced infiltration of outdoor particles as a result of the renovation. In order to remove particles that people generate in their homes more efficiently there is a need for stricter building regulations regarding kitchen extraction hoods and ventilation flows. The challenge here is to create a good balance between indoor air quality and energy savings.
The obtained knowledge in this thesis can be used in developing strategy to minimize exposure to particles indoors. This can be achieved by developing methods of effective removing of particles generated indoors directly at the source and by improving ventilation systems to reduce outdoor infiltration.