Near Highway Pollution: From Research to Action
Phase: NA
Conditions: Air Pollution
Sponsor: UConn Health

 
Effect of Reducing Ambient Traffic-Related Air Pollution on Blood Pressure: A 
Randomized Crossover Trial    NCT04029129 
 
 
 
 
JULY 26, 2023 

 
Objective of the Study: 
Traffic-related air pollution (TRAP) is the predominant contributor to poor air quality in 
developed countries. The 4% of the US population (Boehmer et al., 2013) who reside 
near busy roadways, as well as those who travel on them, are exposed to elevated 
concentrations of TRAP – a mixture of gases and particulate matter (PM) enriched in 
fuel combustion byproducts, as well as no tailpipe emissions. These exposures can be 
an order of magnitude higher than urban background exposure (Karner et al., 2010). 
Both residential proximity to roadways and short-term exposures from time spent on 
roadways have been associated with adverse cardiovascular indicators and outcomes 
(Brook et al., 2010a). Peters et al (Peters et al., 2004) showed in a case-crossover 
study that exposure to TRAP was associated with myocardial infarction within 1 to 2 
hours. Ghosh et al (Ghosh et al., 2016) reported that residential proximity to major 
roadways and traffic density were responsible for 2.4% and 6.8% of coronary heart 
disease and deaths, respectively, in Los Angeles and neighboring counties. Living near 
roadways has also been linked to increased prevalence of hypertension and increases 
in blood pressure (BP) (Brook et al., 2010b; Cai et al., 2016). 
Controlled exposure to diesel particles and concentrated ambient fine PM (PM2.5; 
defined as particles with aerodynamic diameter <2.5 µm) from near-roadway locations 
has been shown to raise BP within 2 hours (Bellavia et al., 2013; Cosselman et al., 
2012; Mills et al., 2011; Urch et al., 2005).There is also evidence that the particulate 

component of TRAP is the causal agent. For example, Mills et al  (Mills et al., 2011) 
reported that diesel exhaust exposure impaired vascular function but that filtered diesel 
exhaust (that retained its gaseous components) or pure carbon nano-PM did not. 
Similarly, Cosselman et al  (Cosselman et al., 2012) reported a rapid increase in systolic 
BP (SBP; peaking 30–60 minutes from the start of exposure) but not diastolic BP (DBP) 
or heart rate (HR), upon exposure to diesel exhaust. In contrast, studies of the effects of 
short-term exposures to gaseous components of TRAP mixtures on cardiovascular 
function and BP are scarce. In the few studies we were able to find, the associations 
with adverse effects for O3 exposure are mixed (Li, 2018), while current evidence does 
not support an adverse acute effect of NO2 on vascular function (Langrish et al., 2010). 
There have been over 90 studies assessing short-term effects of ambient ultrafine 
particles (UFPs; particles with aerodynamic diameter <100 nm and a component of 
TRAP) on cardiovascular indicators and outcomes (Ohlwein et al., 2019). There are 
also a growing number of long-term studies of UFP and health, with a particular focus 
on residential proximity to roadways, but these studies have reported mixed findings in 
terms of associations with cardiovascular health (Aguilera et al., 2016; Lane et al., 
2016). Relatively few have studied short-term effects of TRAP, or its specific 
components such as soot (or black carbon [BC]) and UFP, on BP (Magalhaes et al., 
2018). 
It is difficult to draw causal relationships between TRAP and BP because of the 
observational nature of epidemiological studies, which include uncontrolled 
confounders, such as noise (Aguilera et al., 2016; Brauner et al., 2008; Brugge & Goble, 

2002; Lane et al., 2016; Magalhaes et al., 2018; Morishita et al., 2018; Ohlwein et al., 
2019) and exposures are often estimated based on central-site measurements, which 
introduces exposure error. Controlled exposure studies offer the potential to test 
associations under highly scripted and characterized conditions. However, studies with 
exposure concentrations that are realistically encountered proximate to highways are 
lacking. Furthermore, evaluation of the effectiveness of strategies to mitigate TRAP 
exposures, such as use of commercially available, stand-alone HEPA filtration, have, to 
date, shown limited benefits on BP and peripheral blood inflammation (Brauner et al., 
2008; Brugge & Goble, 2002; Morishita et al., 2018). Morishita et al (Morishita et al., 
2018) found reductions in SBP in older adults living in an urban area with 3-day use of 
HEPA filtration, which also reduced indoor PM2.5, but PM2.5 is not a marker of TRAP 
emissions. Therefore, the aim of our study was to assess whether reducing TRAP 
exposure by limiting indoor infiltration and deploying HEPA filtration for a short duration 
could have a beneficial effect on BP. 
Methods: 
Study Design and Participants 
The study design was a 3-exposure, 3-period crossover trial in which participants 
attended three 2-hour-long exposure sessions (1 each for low, medium, and high 
exposure in random order), all on same day of the week, separated by 1-week washout 
periods. Twenty-four sessions of each type were held, and participants were assigned 
to the exposure sessions using a Latin square design. Two to 4 participants attended 
each exposure session. 

The inclusion criteria were age of 40 to 75 years and speaking either English or 
Chinese. The exclusion criteria were self-reported history of a major cardiovascular 
outcome (including myocardial infarction, stroke, and angina), other serious health 
problems (chronic obstructive lung disease or current asthma), taking antihypertensive 
medications, smoking or living with a smoker, cognitive impairment, working at a job 
with high-combustion exposure (eg, taxi/truck driver and restaurant cook), and high-
combustion exposure in the preceding 24 hours (such as driving on a highway). The 
Tufts University Social Sciences institutional review board approved the study protocol. 
Participants were instructed to avoid caffeine, to not drive, and to avoid exposure to 
traffic other than what was necessary to get from their home to the study location on the 
morning of exposures. Participants were preferentially recruited from neighborhoods 
close to the study locations that have outdoor TRAP levels similar to the exposure 
session location. Most participants walked; however, a few were transported to short 
distances in gasoline-powered cars driven with the in-vehicle air recirculation fan turned 
on to limit exposure before arrival. Before the first exposure, participants were oriented 
to what to expect during the sessions to reduce stress reactions. Exposure session type 
was not disclosed to the participants. 
Participants arrived 15 to 20 minutes before the start of each exposure session. Upon 
arrival, they completed a questionnaire that assessed exposures that could potentially 
affect their BP. Measures included the validated 16-item Perceived Stress Scale,20 self-
reported taking of any medications (day and hour last taken), location and activity for 
each hour in the preceding 48 hours, consumption of caffeine or alcohol past midnight 

the previous night, and vigorous physical activity in the last 24 hours by hour. If an 
exposure of concern was identified, which was rare, the participant was rescheduled. 
 
Air Quality and Outcome Measures and Exposure Room Configuration 
Air Quality Measurements 
Particle number concentrations (PNC) and black carbon (BC) concentrations were 
monitored continuously. PNC was measured using a water-based condensation particle 
counter (TSI, Inc; model 3873; d50=7 nm) at 1-second resolution and BC concentration 
was measured with an aethalometer (Magee Scientific; model AE16) at 1-minute 
resolution. For the first half of the sessions, temperature was measured centrally in the 
room, and for the latter half of the sessions, temperature was measured directly 
adjacent to each study participant with a temperature logger (Onset Computer 
Corporation; model HOBO UX100-003).  
Particle Size Distribution 
Particle size distributions in a subset of exposure sessions (subset session count: low, 
8; medium, 9; and high, 7) were measured with a Scanning Electrical Mobility 
Spectrometer (Brechtel Manufacturing, Inc). The particle counter operated with butyl 
alcohol as the condensing fluid and to prevent detectable butyl alcohol scent, the 
particle counter was enclosed in an airtight box that was vented outdoors. The Scanning 
Electrical Mobility Spectrometer measured particles between 5 and 600 nm in 59 size 

bins, executing a scan across the full-size range every 75 seconds. Average particle 
size distributions were generated for high-, medium-, and low-exposure sessions by 
averaging all scans over the 2-hour exposure session duration and then averaging the 
same exposure types. 
BP and HR Measurement 
Participants were seated throughout the questionnaire (15 minutes) and exposure 
sessions (2 hours). During exposure, participants wore sound canceling headphones 
(Bose noise canceling wireless headphones), were instructed to refrain from doing 
anything, including reading or meditation, and asked not to fall asleep. If it appeared 
that a participant was falling asleep, which was rare (<5 instances), they were gently 
tapped and awakened. An ambulatory BP monitor (Suntech Oscar; Table S1) was 
placed on the dominant arm of participants and began recording at the start of each 
exposure session to record SBP, DBP, and HR every 10 minutes. 
Exposure Room Configuration 
Exposure sessions were held at 2 near-highway locations (shown in Figure S1) where 
ambient air was enriched in TRAP. Most of the participants (n=63) attended exposure 
sessions in a ground-floor community room of a low-income housing development 
located near the intersection of I-90 and I-93 (average weekday daily traffic, 240 000 
vehicles/day) in Chinatown, Boston. For the other 14 participants, sessions were 
conducted at another community room (on the ground floor in a multistory building) 
located in Somerville, MA, which was adjacent to I-93 and R38 (average weekday daily 

traffic, 215 000 vehicles/day). Sessions were conducted in the morning (during 06:00–
09:00 hours) to coincide with rush-hour traffic and lower atmospheric dispersion, that is, 
when TRAP concentrations were expected to be the highest (verified by diurnal trends 
measured at a vicinal near-highway regulatory monitoring site [ID: 25-025-0044]). 
Three levels of exposure—low, medium, and high—were attained by varying the degree 
of ventilation (and thereby infiltration) and by increasing or decreasing the number or 
blower setting of HEPA filtration units. Two brands of HEPA air filters were used at both 
locations: IQ Air Health Pro (1 unit) and Austin Air Healthmate 400 (up to 7 units with 4-
stage filter, which also contained granular activated carbon and zeolite). 
Based on air quality measurements before the exposure sessions, we determined that a 
concentration of ≈20 000 to 30 000 particles/cm3 could be expected during the high-
exposure sessions and 10-fold lower concentrations could be attained in the low-
exposure sessions. We targeted 33-50% lower than high-exposure concentrations, that 
is, 10 000 particles/cm3, for the medium-exposure sessions. 
During low exposure, infiltration of outdoor air into the room was minimized by closing 
doors and sealing the cracks underneath with duct tape. Neither exposure room had 
operable windows. Filtration of TRAP was maximized by continually running air filters (4 
units at the Somerville location and 8 units at the larger Chinatown location). 
During medium exposure, infiltration was restricted by closing doors, but cracks 
underneath were left unsealed. Filtration was optimized to maintain indoor PNC in the 
8000- to 12 000-particles/cm3 range by using 0 to 4 air filters. 

During high exposure, infiltration was maximized by cross-ventilating the rooms by 
partially opening the doors that connected the room directly to outdoors and in the 
Somerville location, by adding a fan in addition to door opening (since cross-ventilation 
was not an option in that room). Air filters were not used during high exposures. 
Both community rooms were located within a block of restaurants, which resulted in a 
noticeable odor during the last 30 to 45 minutes of some sessions when restaurants 
began cooking. Odor was only perceivable in some of the high-exposure sessions 
because this configuration allowed odors into the rooms. 
Statistical Analysis Plan: 
A linear mixed model was used to analyze the 3-exposure, 3-period crossover design. 
The primary outcome measure is the change in systolic blood pressure from the 
beginning of each exposure session. Before calculating change scores in each 
exposure session, each participant’s systolic blood pressure measurements was 
smoothed using a 3-time-period, centered moving average. A random intercept was 
included in the linear mixed model to account for the interdependence of the 
observations that arise by each participant attending each exposure session. In 
addition, the model included baseline systolic blood pressure, mean room temperature 
for each session, and 2 time-varying covariates, particle number count and black carbon 
concentrations. A Toeplitz covariance structure was used to characterize the 
interdependence of the observations over time. Period and carryover effects was 
assessed in the linear mixed model. The mean dose-response effect of filtration was 
assessed for statistical significance using a linear contrast. An identical approach was 

used to analyze diastolic blood pressure and heart rate. Particle number count and 
black carbon concentrations was also analyzed using a linear mixed model with 
average room temperature as a fixed covariate and a Toeplitz covariance structure. 
 
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