Vascular Function and Short-Term Exposure to Fine Particulate Air Pollution

ABSTRACT

Exposure to fine particulate air pollution has been implicated as a risk factor for cardiopulmonary disease and mortality. Proposed biological pathways imply that particle-induced pulmonary and systemic inflammation play a role in activating the vascular endothelium and altering vascular function. Potential effects of fine particulate pollution on vascular function are explored using controlled chamber exposure and uncontrolled ambient exposure. Research subjects included four panels with a total of 26 healthy nonsmoking young adults. On two study visits, at least 7 days apart, subjects spent 3 hr in a controlled-exposure chamber exposed to 150–200 μg/m³ of fine particles generated from coal or wood combustion and 3 hr in a clean room, with exposure and nonexposure periods alternated between visits. Baseline, postexposure, and post-clean room reactive hyperemia–peripheral arterial tonometry (RH-PAT) was conducted. A microvascular responsiveness index, defined as the log of the RH-PAT ratio, was calculated. There was no contemporaneous vascular response to the few hours of controlled exposure. Declines in vascular response were associated with elevated ambient exposures for the previous 2 days, especially for female subjects. Cumulative exposure to real-life fine particulate pollution may affect vascular function. More research is needed to determine the roles of age and gender, the effect of pollution sources, the importance of cumulative exposure over a few days versus a few hours, and the lag time between exposure and response.

IMPLICATIONS

This study found no contemporaneous vascular response in healthy young adults to exposure to a few hours of generated fine particulate matter in a controlled experimental setting. Two-day uncontrolled ambient fine particulate pollution at much lower concentrations was associated with small but significant changes in vascular function, primarily for female subjects only. Clearly, additional research is needed to establish the potential effect of cumulative exposure over a few days versus a few hours and the lag time between exposure and response.

INTRODUCTION

Exposure to fine particulate matter air pollution (particulate matter with an aerodynamic diameter less than or equal to a 2.5 μm cut point, PM_2.5) has been implicated as a risk factor for cardiopulmonary disease and mortality.1–6 There are continued efforts to explore the pathophysiological pathways and biological mechanisms that link PM_2.5 exposure with cardiopulmonary disease and mortality. Recent research has linked particulate-related air pollution to pulmonary and systemic inflammation.7–11 Low-to-moderate-grade inflammation induced by long-term chronic PM_2.5 exposure may initiate and/or accelerate atherosclerosis.4,12–14 Short-term exposure and related acute inflammation may contribute to acute thrombotic complications of atherosclerosis, thus increasing the risk of making atherosclerotic plaques more vulnerable to rupture, clotting, and precipitating acute cardiovascular15–18 or cerebrovascular events.19,20 These proposed biological pathways imply that PM_2.5-induced pulmonary inflammation plays a role in activating the vascular endothelium. Altered endothelial or vascular function plays a primary role underlying cardiovascular disease and is an important PM_2.5-related mechanism.

There have been only a few efforts to directly explore the associations of short-term PM_2.5 exposure with sub-clinical measures of vasculature function because of the difficulty of obtaining adequate repeated physiologic measures. The ability of arteries to dilate in response to appropriate stimuli is a primary feature of vascular function. One measurement approach uses high-resolution brachial artery ultrasound to evaluate flow-mediated reactivity by comparing brachial artery diameter before and after flow mediation using a blood pressure cuff. Using this approach, Brook and colleagues21 observed that PM_2.5 and ozone exposures over 2 hr induced arterial vasoconstriction in nonsmoking healthy adults, and O'Neill and colleagues22 observed that particulate exposure was associated with impaired vascular reactivity in diabetics.

An alternative, noninvasive approach, reactive hyperemia–peripheral arterial tonometry (RH-PAT), has been developed to assess vascular function.23–25 Recently, and before the completion of this study, Bräuner and colleagues reported the results of two studies of vascular function that were also based on RH-PAT measures.26,27 They reported that reductions in 48-hr PM_2.5 due to filtering air in subjects' homes resulted in improved vascular function in aged subjects.26 However, they found no relationship between traffic-related particles and vascular function in young healthy individuals.27

The primary hypothesis of this study was that approximately 3 hr of elevated exposure to PM_2.5 in a controlled-exposure chamber would result in subclinical but measurable declines in vascular function on the basis of RH-PAT. On the basis of the results reported by Bräuner and colleagues,26,27 a secondary hypothesis was included: that vascular function was associated with previous 2-day ambient PM_2.5 exposure.

METHODS

Research Subjects and Participant Protocol

Research subjects included a total of 26 healthy, non-smoking young adults between 18 and 25 years of age living in Utah Valley. Exclusion criteria included the following:

1.	Being unwilling to participate and/or sign the consent form
2.	Body weight of less than 110 lb
3.	Health problems that precluded participation, including latex allergy; lack of two healthy hands and arms; any known chronic pulmonary or cardiac disease; current cold, flu, or other infectious illness; chronic renal failure; Parkinsonism; alcohol abuse; mental illness; bleeding disorders; pregnancy; and past or current history of hepatitis, AIDS, or HIV
4.	Currently living, working, or attending school with exposure to environmental tobacco smoke Participants were fully informed about the study and the study protocol. They then signed informed consent forms and completed an intake questionnaire.

Four panels of six or seven participants were formed. For two prearranged study visits at least 7 days apart, participants in a given panel arrived at the research site where they spent approximately 7 hr completing the study protocol illustrated in Figure 1. Baseline blood pressure was measured on the first visit. Endothelial function was measured at the beginning of each study period, at the end of 3 hr in a room with controlled elevated pollution exposure, and 3 hr in an unfiltered room without elevated exposure. The exposure and nonexposure periods alternated between the two visits. For panel 1 the visits occurred on February 3 and 17, 2007; for panel 2 on March 3 and 10, 2007; for panel 3 on September 29 and October 13, 2007; and for panel 4 on November 10 and 17, 2007. Participants arrived at 20- to 30-min intervals and rotated through the protocol over a 9-hr period. Participants arrived in the morning (from ∼7:30–10:30 a.m.) and were asked to eat only a light breakfast before arrival. A meal of bagels, cheese, jelly, fruit, and juices was provided for consumption at specified times during the exposure-room and clean-room periods (Figure 1). Subject activities were similar in the exposure and clean room. These activities included reading, using laptop computers, studying, and visiting and related activities. Drinking water and access to restroom facilities were continuously available. The institutional review board for human subjects at Brigham Young University approved the research protocols and consent forms. All controlled exposures occurred at the exposure chamber site on the university campus and ambient exposures occurred during normal activities on campus and in the surrounding community.

Figure 1. Overall schematic of the measurement and exposure protocol for (a) first exposure and (b) second exposure.

Controlled Exposure

Controlled PM_2.5 exposure occurred in an exposure chamber described in more detail elsewhere.28 Combustion-generated aerosol was sampled from the flue line of a standard stove designed for in-home use. After being allowed to burn for at least 15 min, the combustion-generated aerosol was moved through stainless-steel tubing, via an inline fan, through a carbon monoxide (CO) to carbon dioxide (CO₂) catalytic converter before being delivered into a large (∼30 m³) Teflon bag. During the wood smoke exposures, the aerosol was photochemically aged using a conventional ultraviolet (UV)/black lamp system.28 Movement of the combustion aerosol into the exposure chamber, via a dedicated recirculation system, was conducted to maintain the desired concentration. Real-time monitoring of PM_2.5, CO, nitric oxide (NO), nitrogen dioxide (NO₂), and oxides of nitrogen (NO_x) in the Teflon bag and the controlled-exposure room was conducted to assist in proper dilution. PM_2.5 concentrations were controlled at approximately 150–200 μg/m³. CO concentrations during the exposure period averaged less than 9 parts per million (ppm). The exposure chamber was furnished to accommodate up to eight participants with a similar, separate room for nonexposure periods.

Ambient PM_2.5 data were obtained from the Utah Department of Environmental Quality, Division of Air Quality (Salt Lake City, UT), for the central site monitor for Utah County. Monitoring was conducted in accordance with the U.S. Environmental Protection Agency Federal Reference Method.29 Average PM_2.5 concentrations for the previous 2 days for each of the relevant study dates were calculated.

Measuring Vascular Function

The Endo-PAT2000 System (Itamar Medical, Ltd.) was used to assess RH-PAT. As illustrated in Figure 1, baseline-, exposure-, and clean-period measurements were conducted. Principles and applications of RH-PAT have been discussed elsewhere.23–25 Briefly, subjects were seated with hands placed on supports approximately level with the heart. Pneumatic finger probes were placed on each index finger, capping the fingers and applying a uniform pressure field that allowed for measurement of the digital volume changes accompanying pulse waves. Tonometric recordings were conducted continuously for 15 min, including a 5-min equilibration period, a 5-min challenge period with the left arm occluded using a blood pressure cuff, and a 5-min postocclusion period. The hyperemic response (measured as the ratio of the postoccluded vs. baseline amplitude in the hyperemic finger normalized relative to the finger of the control arm [RH-PAT ratio]) was calculated using an operator-independent automated algorithm. The microvascular responsiveness index (MVRI), defined as the log of the RH-PAT ratio, was then calculated and tested against the hypothesis that elevated exposures to controlled short-term air pollution will lower MVRI among individuals.

Statistical Analysis

Data analysis included simple comparative statistics and the estimation of fixed-effects regression models. All MVRI observations were included in the regression analysis. The regression models included a binary variable that indicated exposure in the controlled-exposure chamber. This binary variable equaled zero for baseline- and clean-room observations and equaled 1 for exposure-chamber observations. The regression models also included a variable that was the average of the PM_2.5 concentrations for the previous 2 days for each of the relevant study dates. As such, the exposure chamber indicator variable accounted for within-day differences in experimentally controlled exposures, and the previous 2-day ambient PM_2.5 variable accounted for the cross-study day differences in 2-day lagged ambient exposures. To control for cross-subject differences, subject-specific fixed effects were also included in the models. Furthermore, to control for potential trends through the day during the study period, a diurnal trend variable that simply indicated the order of the day's observation was also included. The analysis was also stratified by gender.

RESULTS

Exposure-chamber concentrations of PM_2.5 and ozone are presented in Table 1. For the coal and wood smoke exposures, the levels of PM_2.5 were similar but ozone exposures were higher during the wood smoke exposure period. Table 1 also presents the mean (and standard deviation) of the MVRI for the baseline measurements, the measurements at the end of 3 hr in the clean room, and the measurements at the end of 3 hr in the exposure chamber. On average, MVRI measured after controlled exposure to coal or wood smoke was not significantly different from MVRI measured at baseline or after 3 hr in a control room.

Table 1. Summary study dates, subject numbers, controlled exposure, and mean MVRI

			Mean Concentrations during Controlled Exposure			Mean MVRI (SEM)
Panels	Study Dates	Number of Subjects (% Female)	Source	PM2.5 (μg/m^3)	Ozone (ppb)	Baseline	Unexposed	Exposed
1 and 2	February 3 and 17, 2007	12 (50)	Coal	171	<10	0.42 (0.26)	0.50 (0.28)	0.53 (0.31)
	March 3 and 10, 2007	–	Smoke	–	–	–	–	–
3 and 4	September 29 and October 13, 2007	14 (57)	Wood	180	55	0.40 (0.16)	0.54 (0.28)	0.50 (0.23)
	November 10 and 17, 2007	–	Smoke	–	–	–	–	–
Total	–	26 (54)	–	–	–	0.41 (0.21)	0.52 (0.28)	0.51 (0.27)
Notes: ppb, parts per billion.

Figure 2 illustrates the association between MVRI and ambient PM_2.5. For each of the four panels, the mean MVRI across all panel subjects was calculated for each of the two visits. Because the previous 2-day ambient exposures were at least somewhat different between the visits, paired visits of the four panels provide four contrasts in ambient exposure. The open and dark circles and triangles represent the four different panels. Although the contrasts in exposure were small, small declines in MVRI were generally observed with increased ambient PM_2.5 expo sure. Larger and more significant declines were generally seen among females in the four panels.

Figure 2. Paired associations between mean MVRI (and standard error bars) and previous 2-day ambient PM_2.5 exposure for (a) all participants and (b) female participants. Mean of all MVRI measures across subjects in each of the four panels calculated for each of the two visits plotted over previous 2-day ambient PM_2.5 for each of the visits.

Table 2 presents the regression results from models that regressed the MVRI on subject-specific fixed effects indicator variables and various combinations of variables indicating exposure chamber, ambient PM_2.5 for the previous 2 days, and a diurnal trend variable for all subjects and stratified by male and female. There are several basic findings. First, there was a significant positive trend in MVRI across the three observations over the course of the approximately 6-hr study period. Secondly, MVRI was not significantly associated with controlled exposure in the exposure chamber. Third, small and marginally statistically significant (P = 0.067) declines in MVRI were associated with increased average ambient PM_2.5 for the previous 2 days. This negative association with ambient PM_2.5 was not observed for male subjects, but it was statistically significant (P = 0.006) for female subjects. The negative association between MVRI and ambient PM_2.5 was insensitive to inclusion of the trend or exposure-chamber variables in the model. Also estimated were regression models that included a variable indicating controlled exposure in the exposure chamber for the previous 3-hr period. These models provided no statistically significant evidence of a carryover or lagged effect of previous period chamber exposure.

Table 2. Regression coefficients from models that regressed MVRI on subject-specific fixed effects indicator variables and various combinations of a variable indicating exposure in the chamber, average local ambient PM_2.5 concentrations for the previous 2 days, and a diurnal trend variable

Model	Variable	All Observations (n = 156)	Female (n = 84)	Male (n = 72)
1	Exposure chamber	−0.012 (0.038)	0.012 (0.048)	−0.040 (0.061)
	Previous 2-day ambient PM2.5 (×10)	−0.136 (0.074) (P = 0.067)	−0.270 (0.094) (P = 0.006)	0.008 (0.114)
	Diurnal trend	0.080 (0.022) (P < 0.001)	0.053 (0.028) (P = 0.065)	0.113 (0.035) (P = 0.002)
2	Exposure chamber	0.048 (0.037)	0.051 (0.047)	0.045 (0.058)
3	Previous 2-day ambient PM2.5 (×10)	−0.136 (0.077) (P = 0.080)	−0.270 (0.096) (P = 0.007)	0.008 (0.122)
4	Previous 2-day ambient PM2.5 (×10)	−0.136 (0.074) (P = 0.066)	−0.270 (0.094) (P = 0.005)	0.008 (0.113)
	Diurnal trend	0.077 (0.020) (P < 0.001)	0.056 (0.025) (P = 0.030)	0.103 (0.031) (P = 0.002)
Notes: Data presented as regression coefficients with standard errors and P values when <0.10 in parentheses.

DISCUSSION

This analysis found no contemporaneous associations between MVRI and 3 hr of controlled exposure to coal or wood combustion PM_2.5 at concentrations between 150 and 200 μg/m³. However, small decreases in MVRI were associated with much smaller elevated concentrations of ambient PM_2.5 for the 2 days before PAT measurements, especially for female participants.

Given the lack of response to relatively high concentrations of PM_2.5 in a controlled setting, the apparent response to much smaller changes in previous 2-day ambient PM_2.5 exposure is intriguing. The ambient aerosols and the aerosols generated for exposure in the chamber are quite different. The ambient PM_2.5 was dominated by aged emissions from diesel and spark-source engines, and most of the PM_2.5 mass was secondary nitrates.30 There are several possible explanations for these findings. First, generated PM_2.5 that participants were exposed to in the controlled setting was not toxic as compared with PM_2.5 from the ambient air. Secondly, the length of exposure in the controlled setting was not long enough to observe changes in vascular function. Third, changes in vascular function lag exposure by more than 3–6 hr but are observable within approximately 2 days. Changes in vascular function may be observable in older and/or less healthy individuals but not in young, healthy subjects. Finally, vascular function as measured by MVRI is not actually associated with exposure to PM_2.5, and the association observed in this analysis with ambient PM_2.5 was spurious.

The recently reported results from another study of air pollution exposure and endothelial/vascular function based on RH-PAT26 are comparable. The effects of indoor exposures on microvascular responsiveness of 21 elderly couples living in Copenhagen, Denmark, were evaluated. The study design was a crossover intervention with 48-hr exposure to filtered and nonfiltered indoor air in the subjects' homes. On average, a reduction in 48-hr PM_2.5 concentrations of 7.9 μg/m³ (4.7 vs. 12.6 μg/m³) was associated with an increase in the RH-PAT ratio from 1.78 to 1.95, or a change in the log of the RH-PAT ratio (MVRI) from 0.577 to 0.668. This reported association corresponds to a change in MVRI per 10 μg/m³ of PM_2.5 change in a 48-hr period equal to -0.115, which is remarkably similar to the −0.136 observed in the study presented here. However, in a related study, no statistically significant differences were observed in young, healthy subjects.27

These results, along with the previously reported results by Braüner and colleagues26 suggest that cumulative exposure of 2 or more days to ambient PM_2.5 pollution may adversely affect vascular function. However, these results provide no evidence of a contemporaneous vascular response to a few hours of controlled exposure. More research is needed to determine the potential roles of age and gender, the impact of pollution sources, the importance of cumulative exposure over a few days versus a few hours, and the lag time between exposure and response.

ACKNOWLEDGMENTS

This study was supported in part by funds from the Mary Lou Fulton Professorship, Brigham Young University, Provo, UT.