Indoor and Outdoor Particulate Matter Exposure of Rural Interior Alaska Residents

To assess the exposure of residents in rural communities in the Yukon Flats to particulate matter of 2.5 μm or less in diameter (PM2.5), both indoor and outdoor concentration observations were carried out from March to September 2019 in Ft. Yukon, Alaska. Indoor concentrations were measured at 0.61 m (breathing level during sleeping) in homes and at 1.52 m heights (breathing level of standing adult) in homes and office/commercial buildings. Air quality was better at both heights in cabins than frame homes both during times with and without surface-based inversions. In frame houses, concentrations were higher at 0.61 m than 1.52 m, while the opposite is true typically for cabins. Differences between shoulder season and summer indoor concentrations in residences were related to changes in heating, subsistence lifestyle and mosquito repellents. In summer, office and commercial buildings, air quality decreased due to increased indoor emissions related to increased use of equipment and mosquito pics as well as more merchandise. During summer indoor concentrations reached unhealthy for sensitive groups to hazardous conditions for extended times that even exceeded the high outdoor concentrations. Due to nearby wildfires, July mean outdoor concentrations were 55.3 μg·m which exceeds the 24-h US National Ambient Air Quality Standard of 35 μg·m. Indoor and outdoor concentrations correlated the strongest with each other for office/commercial buildings, followed by frame houses and cabins. Office/commercial buildings with temperature monitors had one to two orders of magnitude lower concentrations than those without.


Introduction
During the harsh winter conditions of the Yukon Flats as well as during smoke episodes of the fire season in summer, residents of the villages of Eastern Interior of Alaska's Yukon Flats valley spend the majority of their time indoors. Exposure to fine particulate matter of 2.5 μm or less in diameter (PM 2.5 ) is well known as health-adverse and can occur indoors as well as outdoors [1]. Poor indoor air quality was linked to respiratory problems especially in children, cancer, sick-building syndrome, fatigue, headache and about 6% -9% reduced performance of office work, among others [2] [3].
Indoor emissions of PM 2.5 stem from cooking, wood stoves, candles, household and office appliances, smoking, incense, insect repellent coils, cleaning, animals and heating [1]. Fine particulate matter may form by gas-to-particle conversion from precursor gases (e.g. nitrogen oxides, sulfur dioxide, volatile organic compounds) that stem from wood smoke, outgassing of building material, and furniture or vehicle emissions in houses with attached or built-in garages. Wear and tear also contribute to indoor particles. Small amounts of silicates, mold and pollen may exist as well.
Indoor emissions of PM 2.5 have less space to dilute and disperse as compared to emissions outside. Indoor PM 2.5 concentrations also depend on the duration and intensity of ventilation, number of occupants and their activity, airflow as well as pollutant dynamics like first order removal and sorptive interaction processes [4] [5]. The construction type affects both-indoor emissions from outgassing and type of ventilation (natural only, natural and mechanical). Mechanical ventilation encompasses kitchen and bathroom exhaust fans or hoods.
The flow rate differs among systems depending on their purpose, size, building type and volume as well as the system's location. Natural ventilation occurs by air flow through open windows or doors. It is driven by the pressure gradients between inside and outside temperature differences and wind [6]. Natural ventilation varies with weather conditions and season. Its impact on indoor air quality also depends on outdoor air quality.
Particle loss rates by Brownian diffusion, gravitational settling, interception, and impaction vary with particle size [4]. Brownian diffusion dominates for ultrafine particles with diameters smaller than about 0.1 μm. Interception, impaction, and gravitational settling dominate the removal of particles exceeding this diameter [5] [7]. Due to activities by the occupants particles may re-suspend after deposition.
The non-existence of other natural air cleansing mechanisms (e.g. removal by precipitation) contributes to high indoor concentrations unless air-purifier or filter is used. A recent study showed that indoor exposure to emitted PM 2.5 (per unit mass) can exceed that of exposure to outdoor emissions by two to three orders of magnitude [1].
The majority of indoor air quality studies have focused on residences in the contiguous U.S. and Europe. Only few studies exist that examined air quality in S. G. Edwin [8], office or commercial buildings [1] [9] [10]. A recent study in New England revealed that homes with a woodstove had 20.6% higher PM 2.5 concentrations than those without. PM 2.5 concentrations were higher in homes with old stoves, non-EPA-certified stoves, and when burning wet or mixed (vs. dry) wood than those with new EPA-certified stoves burning dry wood [11]. In rural, tribal communities, especially when they are off the road network, old, pot-bellied wood stoves still exist for heating and cooking.
Studies on wildfire smoke impacts mainly focused on urban communities in highly populated areas with distinct firefighting in place [12]. Wildfires are a natural element of the landscape evolution of the Interior and typically occur between May and September in Interior Alaska. Due to the sparse population of the Eastern Interior and undeveloped road network, however, wildfires are watched and only fought actively when they might endanger historic heritage places or burn properties. This so-called "let burn policy" exposes residents to wildfire smoke-often at unhealthy concentrations-over extended periods [13] [ 14]. Furthermore, studies suggested that the frequency and extent of wildfire might increase in the future [15].
To assess the vulnerability of communities in the Yukon Flats, indoor aerosol concentration data were collected in various types of buildings in conjunction with simultaneous outdoor measurements in the largest community of the Yukon Flats (to ensure privacy). The objectives were to assess differences and delays between indoor and outdoor air quality, and to identify ways to reduce exposure.

Network
To accomplish our goals, we conducted a survey of home and business construc-  [13]. Fort Yukon is located on the north bank of the Yukon River in the center of the Yukon Flats.
To monitor the outdoor air quality and weather conditions, we deployed four EPA-suggested equivalent method monitors and meteorological measuring devices strategically outside the city of Ft. Yukon ( Figure 1).
Aerosol indoor PM monitors were located in the five business buildings at  Based on the survey two different home types-cabins and frame houses-were chosen for this study. In Ft. Yukon, cabins are a combination of regional endowed style and modern assembly changes. Frame homes are the common plywood, insulation, and tine structural types normally seen everywhere in the US in rural areas. All homes used woodstoves and furnace with temperature monitor for heating.
Office/commercial buildings with and without monitor were considered as well.

Specifications of the Instruments
A Met One Instruments' BAM-1022, Decagon's EM50 meteorological monitor, Davis cup anemometer, VP-4 ambient temperature/barometric pressure/relative humidity combination sensor were deployed at each of the four outdoor sites. The BAM-1022 were in an enclosed housing to protect the air intake vents from horizontally blown snow and dust, and solar radiation (Figure 2(a)). Air was taken in at 2 m height. BGI VSCC Very Sharp Cut Cyclone (BX-808) particle size separator was set to let only particles equal to 2.5 μm or smaller to pass. The combination sensor recorded temperature, barometric pressure and relative humidity at 2 m height. At 5 m height, precipitation and total in-coming radiation were observed. The Decagon EM50 measured temperature, pressure, and relative humidity at 10 m height. Wind speed and direction were measured at 10 m height as well. Table 1 summarizes the instrument specifications. The real-time measurement interval was 5 min for all devices. Open Journal of Air Pollution  The Dylos DC-1700 indoor particle monitor is a highly accurate class 1 laser particle counter, and complies with 21CFR1040.10 and 11 ( Figure 2(b)). The DC-1700 can display both actual particle concentrations, which relates to the International Standard (ISO) for indoor air (Table 2), as well as mass concentrations, which relates to EPA standards (Table 3). Herein, an indoor aerosol concentration of 1000 μg·m −3 , for instance, would correspond to 100,000 particles of 0.5 micron or smaller in diameter, and would indicate very poor air quality.

Quality Assurance/Quality Control and Data Processing
The data files of each of the outdoor sites were synchronized for time and combined. The same quality assurance/quality control (QA/QC) protocol as described    [13] was applied for both the indoor and outdoor observations. Missing data were tagged as such; false and/or data beyond an instrument's range were discarded and tagged as such.

Data Analysis
To ensure the privacy of the home and business owners, we striped the data from all information that could be used for identification (location, owner, renter, number of occupants, year of construction, etc.). In doing so, we averaged the recorded data over all measurements from homes of same construction and over all offices/commercial buildings with same heating appliance type. The indoor concentrations served to quantify the long-term means of exposure.
For all quantities, we determined the hourly, daily, monthly and seasonal means and standard deviations. We compared the 24-h means of indoor and outdoor PM 2.5 concentrations to the ISO rating scales ( The outdoor concentration and wind direction data at individual sites were analyzed to assess the contributions of emissions from the various sources from Open Journal of Air Pollution within the village and from outside.
To attribute sources, we calculated hourly PM 2.5 concentration means as a function of wind direction using the WHO and EPA recommended algorithm [16] [17] [19].
To examine the relation of indoor and outdoor air quality and its dependence on the weather conditions, we averaged the meteorological and air quality data of the four outdoor PM 2.5 monitors as well. The hourly means over the four outdoor PM 2.5 concentrations were also used to identify surface-based inversions (SBI), their onset and dissipation as well as duration and the air flow in and out of the community following [13] [20]. These hourly means also served to examine differences in the degree of pollution as compared to indoors.

Emission Sources
The In Ft. Yukon, large emission sources for PM 2.5 and its precursor gases stem from burning grade-1 diesel for heating the city office buildings, corporation buildings, stores, businesses, the Tribal Council and Tribal Consortium buildings, regional health clinic and laundry-mat [13]. Sources of silicate aerosols are the city road systems and dust-uptake by wind from river gravel bars [13] [20] [22].
The analysis of the wind direction data and outdoor PM 2.5 concentrations confirmed the findings of [13] regarding the major contributors to local air concentrations.

Meteorological Conditions
Daily means of 10 m height wind speed rarely exceeded 5 m·s −1 (Figure 3(a)). At 10 m height, daily means of relative humidity varied between 33% and 96% and were 59% on average over all days. Daily means of relative humidity at 2 m height ranged between 32% and 95%, but were on average around 57% ( Figure   3(a)). At this height, daily mean temperatures were between −11˚C and 24.5˚C ( Figure 3(a)). At 10 m height, these values were −11.4˚C and 24.9˚C, respectively. Total accumulated solar downward radiation varied between 80 kW·m −2 ·h −1 and 1247 kW·m −2 ·h −1 depending on the day of year and atmospheric conditions including cloudiness and aerosol optical depth (see Figure 3(b)).

Surface-Based Temperature Inversions
Following [20], we used hourly means of the outdoor temperature measurements at 10 m and 2 m height as an indicator for the occurrence of surface-based  temperature inversions during the respective days of the study. For each day the start and end time as well as the duration of the surface inversion were determined (e.g. Figure 4).  All days with complete temperature data had surface-based inversion (e.g. Figure 4). The duration throughout a day varied from March to September. The majority of all SBI occurred through the late night and early morning, local time.
Some multiday SBI occurred in the shoulder season. Occasionally, more than one SBI formed per day. Durations of SBI were typically shorter in April than in the other months due to increased wind speed. Furthermore, the onset of melting led to differential heating and convection broke the SBI.

Outdoor Air Quality
The daily data of hourly means of temperatures and the derived onset, dissipation and duration of inversions were utilized to separate the measured outdoor PM 2.5 concentrations into two categories, namely those (a) occurring during a surface-based inversion (SBI) event and (b) those occurring during hours without an SBI (Table 4).
Following the EPA guidance, we calculated the 24-h mean outdoor PM 2.5 concentrations from hourly mean concentrations. To assess the exposure on a monthly basis we averaged over the daily 24-h means of the respective month (2 nd column in Table 4). Since inversions only existed for parts of the day (see e.g. Figure 4) and started or ended not necessarily to the full hour, means for times with and without inversions were calculated for each month based on the 5-min mean raw data that were kept after the QA/QC. These monthly means for times with and without the presence of SBI are up to about factor 2 higher than the monthly means calculated from 24-h means calculated in accord with the NAAQS ( Table 4).
The difference between the monthly mean of 24-h means and the monthly means for times with or without inversions in the following tables is that the latter two cover not the full amount of hours of the months. This means that they are expressed in terms of means for the times with or without inversions. The monthly mean of 24-h average PM 2.5 concentrations is given by

Indoor Air Quality
On average over all indoor sites, a diurnal course can be detected in office/commercial buildings and at both heights for residences ( Figure 6). Typically, indoor concentrations were higher during the day than at night which broadly coincides times of no SBI and with SBI ( Table 5, Table 6). This finding is expected as people move around in their residences thereby re-suspending particles into the air that had already settled on the floor or other surfaces. Various daytime activities like cooking, smoking or feeding of woodstove could lead   (Table 7). Indoor concentrations at 1.52 m were lowest in July and highest in April and May. On average over all residences, indoor concentrations were higher at 0.61 m than at 1.52 m height (e.g. Figure 6(a)).
Averaged over all residencies, monthly mean PM 2.5 concentrations were 12.5 µg·m −3 , 40.1 µg·m −3 , 75.3 µg·m −3 , and 46.1 µg·m −3 higher at 0.61 m than at 1.52 m in April, May July and August, respectively. This distribution can be partly explained by gravitational settling. The hourly mean values at those heights showed weak correlation (21%). These findings mean that exposure to high concentrations was on average over all residences higher when laying down than when standing or walking around (cf. Table 7).
The exposure at both levels was higher than the median of 6.65 µg·m −3 found by [11] for households with woodstoves in New England. The values of the lower and upper range of mean hourly indoor PM 2.5 concentrations ( Figure 6) were lower than the 112 -416 µg·m −3 observed by [25] in homes in Beijing, China in winter. PM 2.5 concentrations in office/commercial buildings in Ft. Yukon were on average very unhealthy for some hours on a daily basis in June and August and were even hazardous most of July. Exposure of employees, clients and customer was much higher than found for these groups in other places. Measurements in an office building in Guangzhou, China, for instance, showed only 3% and 1% of the time very unhealthy and hazardous condition in JJA [10]. In Dublin, Ireland, indoor PM 2.5 concentrations during working hours were typically below 25 µg·m −3 with highest values in naturally ventilated shops [9].   In Guangzhou and Dublin, for instance, I/O were below 1 [9] [25].

Indoor vs. Outdoor Air Quality
In the analysis of the impact of inversions on indoor concentrations, the question of air exchange arises. To compare indoor and outdoor concentrations of PM 2.5 during times with and without SBI the temporal lag between indoor and outdoor concentrations has to be determined first. We applied a lag-time correlation method [26] at various temporal lags using the means over the hourly outdoor concentrations and those averaged over all hourly indoor concentrations as input data. The method revealed a maximum correlation for a lag-time Open Journal of Air Pollution of 1-hour and 2 hours for residences at 0.61 m and 1.52 m heights (e.g. Figure   7), respectively. It showed a time-lag of 1 hour for office/commercial buildings.
When people are not home in their residents, the floor to door gaps, and other interior-to-exterior connections (e.g. cooking hoods, vents) have a greater influence at floor levels where air settles and is cooler than at 1.52 m. The 1.52 m level has a longer lag-time due to dead air and diffusion time.
Based on these findings and the determined onset and ending times of the SBIs within each 24-hour period (e.g. Figure 4), we adjusted the timing of indoor concentrations to account for the lag-time for the residences at both heights and the office/commercial buildings. Doing so synchronized the indoor concentrations with that outdoors for times with and without inversions.
On average over all residencies, indoor concentrations were higher during times without inversions than during SBIs at 0.61 m height in March, April and September (Table 5). From May to July, indoor concentrations at 0.61 m height were lower during times without inversion than during times with inversions.
Averaged over the few available March days and all residences, the means were These findings can be explained as follows. In April and September, heating is still and already again needed. Woodstoves release PM 2.5 and its precursor gases during operation. Typically, they are used for heating during the day and are fed once more prior to bedtime. Once all wood is burned, these emissions stop.
When indoor temperatures fall below the monitor-set threshold, the furnace turns on. The emissions from diesel heating mostly leave via the chimney. Furthermore, settled particles are not re-suspended when the residents are sleeping.
Consequently, PM 2.5 concentrations were lower during nighttime than daytime.
Since, most inversions occurred at night (cf. Figure 4), indoor concentrations were lower during inversions than no inversion conditions in the heating season (Table 5). People are awake the majority of the time during the day when no surface inversion exists. Consequently, due to their activities PM 2.5 concentration levels were highest indoors during this time.

Relation between Indoor Air Quality and Building Type
To examine the relation between construction types and building use, we averaged the hourly mean indoor PM 2.5 concentrations for each construction type and measurement level. Furthermore, we calculated hourly concentration means for residencies and office building by averaging over all respective sites.
Typically, hourly mean values at 0.61 m and 1.52 m heights correlated weakly (27.4%) and hardly (1.5%) for frame houses and cabins, respectively ( Figure   7(b)). This means that indoor air quality of frame houses is stronger related to outdoor air quality than indoor air quality of cabins. On average, in frame houses, concentrations were higher at 0.61 m than 1.52 m, while the opposite was observed in cabins (cf. Figure 6(b), Figure 8). Daily 24-h mean indoor PM 2.5 concentrations at both heights for the greater part exceeded the NAAQS frequently ( Figure 8).
On average, cabin indoor PM 2.5 concentrations at 0.61 m height were higher during times without inversions than during SBIs ( Table 5). The same is true for frame houses in April and September, while the opposite was true from May to August (Table 5). At 1.52 m height, means of both cabin and frame house indoor PM 2.5 concentrations were higher for times with than without inversions in all months (Table   6). Cabins provided the lowest exposure at both heights (  These insights suggest that in frame houses, sleeping on top-bunks seems to be beneficial for health except during wildfires. Also cabins seem to be superior over frame houses in keeping outside PM 2.5 concentrations at bay. Recall no measurements were made at 0.61 m height in office/commercial buildings because nobody sleeps in them. In April and May, concentrations at 1.52 m were higher in residences than office/commercial buildings both during times with and without SBI (cf. Table 6). At night, office/commercial buildings are closed meaning that particulate matter settled. Consequently, 1.52 m concentrations went down as compared to the times without SBI, i.e. during the day.
The higher concentrations in residences than office/commercial buildings during times without SBI can be explained by the different kind of indoor activities, emissions sources and emission rates at home and work.
In JJA, monthly mean and 24-h mean PM 2.5 concentrations in office/commercial buildings exceeded those in residences (cf. Table 6, Table 7, Figure 8, Figure 9).
For both, concentrations were higher during SBI than during times without SBI ( Table 6). The reasons for this shift in concentration behavior are as follows. In JJA, residents were involved in activities related to the various harvest seasons   showed that using both low sulfur fuel and EPA certified wood stoves reduced the concentrations of PM 2.5 as compared to the old devices used in Fairbanks [28]. Therefore, we examined the impacts of temperature monitors on indoor air quality of office/commercial buildings. All residences had a monitor and burned both wood and diesel, while office/commercial buildings only burned fuel.
In office/commercial buildings without temperature monitor, monthly and 24-h mean PM 2.5 concentrations were up to more than two orders of magnitude higher than in office buildings with monitors during JJA (Table 7, Figure 9).
This means that using a temperature monitor could reduce exposure of em-

Conclusions and Recommendations
The exposure of rural communities in the Yukon Flats to PM 2. While indoor air quality was moderate in March, it decreased towards summer to hazardous levels on monthly mean and also exceeded those observed outside for notable amounts of time for all building types. Typically, concentrations were lower in cabins than frame houses. In frame houses, PM 2.5 concentrations were higher at 0.61 m than 1.52 m, while the opposite was true in cabins.
Based on these findings one has to conclude that 1) The new log cabins in Fort Yukon provide better indoor air quality conditions than modern frame homes; 2) Sleeping on bunk beds would be beneficial for health in frame houses; 3) Yukon Flats communities are exposed to hazardous indoor PM 2.5 concentrations all summer.
On average, there was a one-hour time-lag between changes of indoor and outdoor air quality conditions. Cabins had the lowest correlation between indoor and outdoor PM 2.5 concentrations followed by frame houses and office/commercial buildings with the highest correlation. Therefore, one has to conclude that frame homes have a higher ventilation and air exchange than cabins, and that frequent opening and closing of doors during hours of customer traffic increased air exchange. The additional emissions from increased amounts of merchandise and/or increased use of equipment in office/commercial buildings and mosquito pics caused hazardous PM 2.5 concentrations at breathing level. Implementation of a temperature monitor could reduce indoor PM 2.5 concentrations by about two orders of magnitude. According to a study in Libby, Montana a change-out program for wood and cooking stoves could improve Open Journal of Air Pollution indoor air quality by 53% [29].