Current Issue

Recently, people have become greatly concerned about particulate matter (PM) among the various ambient air pollutants. Environmental standards for PM, especially for PM_2.5 (particulate matter less than 2.5 μm in aerodynamic diameter), have been set in many countries. We have been conducting a PM_2.5 epidemiologic study and evaluating health risks of ambient PM_2.5 in Japan, and presented an outline of the study and results at an international conference (Nakai et al., 2008a, b, c). However, PM sources, such as smoking, exist inside homes as well as outside, and indoor sources may contribute more than outdoor sources to personal exposure levels, because people spend a lot of time inside homes and buildings. In this paper, we briefly review the results of a personal exposure assessment of an epidemiologic study in Japan from the view point of the indoor environment and discuss the relationship between indoor and outdoor PM concentrations for PM exposure assessment. We also describe the results of other studies on indoor PM_2.5 measurements.

The Ministry of the Environment, Government of Japan, launched epidemiologic and toxicological studies to investigate the relationship between ambient PM_2.5 and its health effects. The studies started in 2001. The design of the epidemiologic study was based on the comparison of health between polluted and unpolluted areas to investigate PM’s chronic effects, such as respiratory symptoms and lung function, or between days to examine acute effects, such as daily mortality and morbidity. In order to validate the study it is necessary to evaluate whether a concentration of PM_2.5 at a monitoring site is representative of personal exposure levels around the site. First, we selected a PM sampler suitable for the study, and then measured PM_2.5 personal exposure levels, and concentrations inside and outside residences.

In general, the objectives for measuring indoor and outdoor PM concentrations and for investigating the relationship between indoor and outdoor environments can be summarized as follows:

• ∙To investigate the possibility of outdoor concentration as a personal exposure surrogate;
• ∙To find and evaluate the modifier(s) for the health effects of ambient air pollution;
• ∙To find specific indoor PM sources; and
• ∙To investigate the distribution of PM concentrations among houses as well as among areas.

The subject persons/residences of PM_2.5 measurement were selected form among the non-smoking persons/houses out of the study subjects. Therefore, it would be difficult to discuss the general indoor environment in Japan from the results of this study.

Twenty-four-hour indoor and outdoor PM measurements were taken simultaneously using an impactor over seven consecutive days during each season: spring (March-May 2003), summer (June-August 2005), autumn (September-November 2004), and winter (December 2003-February 2004). Almost 20 residences without tobacco smoke from each study area (seven cities across Japan) were selected as the subjects of the indoor and outdoor measurements. A sampler for indoor measurement was set on the top of a cabinet/television in a living/children’s room, and one for outdoor measurement was set under the eaves. Personal PM_2.5 and PM_10-2.5 (particles smaller than 10 μm in diameter but larger than PM_2.5) exposure levels were measured in summer and in autumn for almost ten non-smoking parents/guardians of the subject children of the cohort study in each area.

The objective in the measurement design for the air-pollution epidemiologic study was mainly to compare several areas suspected of having different ambient PM_2.5 levels. The study areas were selected based on monitoring data of certain pollutants. Fig. 1 shows the study areas (areas A-G). Unfortunately for the study, but fortunately for the air environment in Japan, outdoor PM_2.5 concentration levels were not so different between the areas. The highest mean concentration from the annual means for 2001 to 2005 was around 25 μg/m³. This is not so high (data not shown), and may reflect the fact that some pollution reduction strategies, such as the use of diesel exhaust filter equipment, have worked well.

Fig. 1. 
Study area of a PM_2.5 epidemiologic study in Japan (Nakai et al., 2008a).

Subject residences were distributed mostly within a 5-km radius from each monitoring site, except in area C. Area C (City C) does not have a monitoring station; therefore, data from the monitoring station in area B, which is next to area C, was used as the ambient concentration of area C. Residences in area C were almost all within a 10-km radius from the monitoring station in area B.

An ATPS-20H impactor (Sibata Scientific Technology Ltd.) was selected as a sampler for the epidemiologic study from among three types of samplers (ATPS, PEM (SKC), and Spiral (SKC)), because its two-stage filters enable the simultaneous collection of PM_2.5 and PM_10-2.5, and because it has a low-volume (1.5 L/min) and low-noise sampling, which is important for the survey inside homes, and finally because it has a lower price than the other samplers. Of course, the characteristic of particle penetration through the inlet of the ATPS-20H fits well with WINS (well impactor ninety-six; Peters et al., 2001), which was designed and calibrated to serve as a particle size separation device for the EPA reference method sampler for particulate matter under 2.5 μm aerodynamic diameter (Fig. 2).

Fig. 2. 
Size-separation characteristics of ATPS sampler (Nakai et al., 2008b).

The EPA well impactor ninety-six (WINS) was designed and calibrated to serve as a particle size separation device for the EPA reference method sampler for particulate matter under 2.5 μm aerodynamic diameter.

Personal exposure levels were measured during two measurement periods (autumn and summer). Although indoor PM_2.5 concentrations in some cities were higher than the outdoor concentrations in the autumn and winter, the concentrations outside and inside homes and personal exposure levels were similar in most areas (Table 1). For example, PM_2.5 concentrations at monitoring station A were 41.1 μg/m³ in the autumn and 29.0 μg/m³ in the summer; outdoor PM_2.5 levels were 43.8 μg/m³ in the autumn and 28.6 μg/m³ in the summer; indoor levels were 34.3 μg/m³ in the autumn and 26.4 μg/m³ in the summer; and personal exposure levels were 37.0 μg/m³ in the autumn and 25.3 μg/m³ in the summer. Daily variations in indoor concentrations and personal exposure were also similar to outdoor concentrations in all areas (Fig. 3).

Fig. 3. 
Daily variation of PM_2.5 concentrations (Nakai et al., 2008b).

It should be possible to estimate the personal PM_2.5 exposure level and indoor PM_2.5 concentrations from the outdoor PM_2.5 concentration when no PM sources exist inside a residence. But, PM_10-2.5 concentrations outside the home were lower than inside, and personal exposure levels were the highest. An indoor PM concentration that is higher than the outdoor concentration suggests that there are some unknown sources for PM inside houses, even in no-smoking houses. Unfortunately, we could not find the sources in this study. It will be necessary to find indoor PM sources and determine their contribution to indoor concentrations and personal exposure in order to improve PM pollution.

In PM epidemiologic studies, chronic and acute health effects of ambient PM pollution have been evaluated, and indoor pollution has been considered as a confounding factor or an effect modifier in chronic health effect studies but not discussed in acute health effect studies.

Here, we would like to introduce the results of one acute health effect study (Ma et al., 2008), and describe the role of indoor pollution on health. The study was conducted at a hospital in Chiba Prefecture. The relationship between the change in peak expiratory flow (PEF) and indoor and outdoor PM_{2.5 (LD)} concentrations was investigated in asthmatic inpatient children. PM_{2.5 (LD)} means PM_2.5 measured by a dust monitor with a laser diode (LD-3K, Sibata Scientific Technology Inc.). According to Ma et al., the concentration of indoor PM_{2.5 (LD)} showed weak correlation with outdoor PM_{2.5 (LD)} concentration and no correlation with the concentration of stationary-site PM_2.5.

The change in PEF was significantly associated with outdoor PM_{2.5 (LD)} concentration (Ma et al., 2008). However, the changes were smaller than that seen for indoor PM_{2.5 (LD)} (Fig. 5). It was concluded that the effects of indoor PM_{2.5 (LD)} were more marked than those of outdoor PM_{2.5 (LD)} or stationary-site PM_2.5. It was also revealed that the time patterns of indoor and outdoor PM concentrations were not same and that the sources of indoor PM might be different from those of outdoor PM; but, it was impossible to find any typical indoor sources, such as smoking in the hospital. Indoor PM pollution and its sources will have to be studied to clarify the effects of indoor PM on children’s health.

Fig. 4. 
Relationship between indoor and outdoor PM_2.5 concentrations and personal exposure levels (Nakai et al., 2008b).

Fig. 5. 
Changes in peak expiratory flow (PEF) in relation to the concentration of PM_2.5 for every 24-hour period up to 3 days before the measurement (Ma et al., 2008).

People may have another concern-that is, whether indoor and outdoor PM_2.5 concentrations close to heavy traffic trunk roads are higher than at points far from the roadside. We investigated indoor PM_2.5 concentrations related to outdoor concentrations. One result is shown here, even though the study was conducted in the 1980s (Ono et al., 2008). Outdoor PM_2.5 concentrations were higher than those of today. Several factors, such as automobile engines, house structure (air tightness), and the use of air-conditioners, should have changed since then. Therefore, the current situation concerning the outdoor and indoor PM environment should differ widely from the reported results from the 1980s. In general, outdoor PM_2.5 and PM₁₀ concentrations were higher in roadside areas than backyards. However, the concentrations of PM_2.5 and PM₁₀ in smokers’ houses were significantly higher than those in non-smokers’ houses (Ono et al., 2008), and the contribution of smoking to indoor PM_2.5 concentrations was bigger than that of the outdoor air due to automobile exhaust (Fig. 6). It was also reported that approximately 50-80% of the indoor PM_2.5 mass was associated with cigarette smoke in winter, and the contribution of tobacco in summer was 70-80% based on a receptor model, although the contribution varied among the data sets (Nitta et al., 1994).

Fig. 6. 
Indoor PM_2.5 levels stratified by zone from the roadside and smoking residences (from Ono, 1989; Nakai, 2003; and Ono, 2008)

Particulate matter mentioned in this paper was related to outdoor PM sources, mainly automobile exhaust. We would like to show another type of PM pollution, namely bio-aerosol. We investigated the longitudinal change of airborne mite-allergen levels in one home, although the relationship between indoor and outdoor was not considered. Fig. 7 shows weekly indoor mite allergen levels. The highest mite allergen level (Der 1) in the living room was 25.7 pg/m³ during November, and the highest monthly PM₁₀ level was observed during October. However, no relationship was observed between the weekly mean level of indoor airborne mite allergen (Der 1) and indoor PM₁₀ (Nakai et al., 1999). Seasonal changes of clothes and bedding, which might be different factors from the ones for PM_2.5 and PM₁₀ pollution, should be considered to investigate bio-aerosol pollution.

Fig. 7. 
Weekly indoor airborne mite allergen levels (Der 1) (Nakai et al., 1999)

In this paper, we tried to describe indoor PM pollution from the view of the relationship between indoor and outdoor pollution, because people spend more time indoors than outdoors. However, they also spend time outdoors, and so outdoor PM pollution should remain a main concern in general. It was revealed that outdoor PM pollution contributes to personal exposure in the case of no PM sources inside homes. However, the contribution of indoor sources on indoor air pollution is high, and in some cases higher than outdoor sources. The health effects of indoor PM are not yet clear, although PEF among asthmatic impatient children was affected by indoor PM. The relationship between indoor PM and other pollutants, such as other combustion products, and biological contaminants has to be considered to evaluate their health effects.