Near Highway Pollutants in Motor Vehicle Exhaust

Michelle Hofmann, MD, MPH

Overview

• Background

• Pollutants of concern

• Air pollutant gradients near highways

• Variables to consider when doing studies

• Health effects

• Key points for AWG to consider

Background

11% of US households are located within 100 meters of 4 lane highways

Near highway pollutants may pose greater health risks than ambient air pollutants

Motor Vehicle Pollutants of Concern

• Ultrafine particles (UFP)

• Black carbon (BC) or “soot carbon”

• Nitrogen oxides (NOx)

• Carbon monoxide (CO)

• Particle-bound polycyclic aromatic hydrocarbons (PPAH)

Ultrafine Particles

• Aerodynamic diameter of 0.005 to 0.1 microns

• Formed by condensation of hot vapors in tailpipe emissions

• Can grow in size by condensation (PM2.5, PM10) or shrink in size by evaporation

• Inflammatory response after PM exposure plays central role in subsequent health impacts

• Smaller particles appear to have greatest impact

• Number of particles appear to be more important than massSource: Hunt et al. Toxicologic and epidemiologic clues from the

characterization of the 1952 London smog fine particulate matter in archival autopsy lung tissues. Environ Health Perspect 111:1209-1214 (2003).

Black Carbon or “Soot Carbon”

• Type of particulate matter• Impure form of elemental

carbon with graphite-like structure

• Product of incomplete combustion of fossil fuels, biofuels and biomass

• Important light-absorbing aerosol species – Runner up to CO2 in

emissions resulting in global climate change

• Health effects related to contribution it makes to PM

Nitrogen Oxides• Binary compounds

containing nitrogen and oxygen

• Result of combustion processes, particularly at high heat

• Highly reactive• Principal health effects on

the lung– Direct effects– Contributor to ozone air

pollution

Carbon Monoxide• Colorless, tasteless, odorless,

nonirritating, flammable and poisonous gas

• Forms as a result of incomplete combustion of fuel

• Major air pollutant of concern since mid 1960s

• 90% reduction in emissions since widespread implementation of catalytic converters

• Acute clinical poisoning does not occur as a result of exposure to ambient concentrations of CO near highways

• Enters bloodstream through lungs and forms carboxyhemoglobin, which inhibits the blood’s capacity to carry oxygen

Particle-Bound Polycyclic Aromatic Hydrocarbons (PPAH)

• Formed by incomplete combustion of organic materials, such as wood or fossil fuels

• Made up of three or more benzene rings

• Low vapor pressure results in their adsorption to PM in atmosphere

• Benzene is known human carcinogen

• Benzo[a]pyrene (B[a]P) most studied – One study found B[a]P levels

2-fold higher both indoor and outdoor in high vs. low traffic areas

How Do We Measure Near Roadway Pollutants?

• Particle counters (UFP)• Analyzers that measure

light absorption (BC and CO)

• Analyzers that measure chemiluminescence (NOx)

• By weight (PM2.5 and PM10)

• Gas chromatography (PPAH and VOC)

Air Pollutant Gradients Near Highways

• Shi et al: UFP (particle number concentration) decreased nearly 5-fold within 30 meters of major roadway (>30,000 vehicles/day)

• Zhu et al: CO, BC, and UFP decreased exponentially between 17 and 150 meters downwind from highway– At 300 meters, UFP same

as at upwind site Source: Zhu et al. Study of ultrafine particles near a major highway with heavy-duty diesel traffic. Atmospheric Environment 2002;36:4323-4335.

Normalized particle number concentration for different size ranges as a function of distance from a highway

Factors Affecting Air Pollutant Gradients

• Condensation/evaporation/dilution– Freshly emitted UFP may differ in chemical composition from

UFP that has undergone atmospheric transformation during transport to downwind locations

• Wind speed and direction– Hitchins et al: Distance from highway at which UFP decreased

50% ranged from 100 to 375 meters based on wind speed and direction

• Contribution of pollutants from other nearby roadways– Morawska et al: No difference in UFP along horizontal and

vertical transects 15-200 meters from highway presumably due to mixing of highway pollutants with emissions form traffic on nearby roads

KEY POINT: Proximity is Key

Much higher exposures to traffic-related air pollutants occur within 30 meters as compared to >200 meters

Some Pollutants of Concern NOT Necessarily

Influenced by being Near-Roadway • PM2.5 and PM10

– Fischer et al: Not specific indicators of traffic-related air pollution

– Roorda-Knape et al: Concentrations did not change with distance

– Janssen et al: PM2.5 (but not PM10) increased with truck traffic and decreased with distance from highway

• Benzene– Roorda-Knape et al: Concentrations did not change

with distance– Janssen et al: Concentrations did not change with

distance

KEY POINT: We Don’t Need to Measure Everything

Because of their common source—vehicle emissions—

UFP, NOx, BC and CO are highly correlated

Variables to Consider in Studying Near Highway Pollutants

• Type of highway– Multi-lane high-speed roadway with restricted access– Four-lane (2 in each direction) variable-speed roadways with

unrestricted access• Types and amounts of vehicles using highways

– Time of day– Day of week– Use restrictions for certain classes of vehicles– Average age and state of repair of vehicles– Fraction of vehicles that burn diesel and gasoline– Fraction of vehicles that have catalytic converters

• Driving conditions• Fuel chemistry• Meterology

Things to Consider for Future Study

• More study needs to be done to confirm existing findings

• Studies that integrate exposure at school, home and during commuting

• Studies that adequately address SES as a potential confounder

Questions?

Cardiovascular Health Effects• Strongest associations with particulates, particularly UFP

– In multiple studies, decreased heart rate variability (HRV) strongest for smallest fraction size studied

– Tonne et al: 5% increase in acute MI associated with living within 100 meters of major roadway

– Lipfert et al: Traffic density better predictor of mortality than ambient air pollution

– Also associations with CO, SO2, NO2 and BC• Mechanisms

– In general• Pulmonary and systemic inflammation

– Long term, repeated increases in inflammation promote atherosclerosis• Altered cardiac autonomic function

– Decreased HRV promotes acute MI

– UFP likely has greater potential for inducing inflammation • Higher particle numbers• More efficient deposition in lungs• Surface chemistry that allows for greater adsorption of other toxic air

pollutants (e.g., PPAH)• High cytotoxic reactive oxygen species activity

Asthma and Highway Exposures

• Early studies used large geographic areas or overall traffic in vicinity and found no association

• More recent studies with increasingly narrow definitions of proximity to traffic have all found associations between asthma or wheezing and living very close to high volume roadways– Controlled for confounders like housing conditions, ETS, SES, age, sex,

atopy• Other findings

– Girls appear to be at greater risk than boys– Exposure in the first 2 years of life may be key– Ambient air pollutants associated with asthma: CO, NOx, PM2.5, BC– Ambient air monitoring at residence substantially increases power to

detect association of asthma with highway exposures• Modeling of proportion of near highway pollutants attributable to

traffic may be better predictors than proximity– Elemental carbon attributable to traffic based on ambient PM2.5

monitoring and association with infant wheezing– NO2, PM2.5 and soot and association with asthma and food allergies

Pediatric Lung Function and Highway Exposures

• Multiple studies with effects on chronic lung development as well as acute lung function

• Gauderman et al: decreased FEV1 associated with PM10, NO2, PM2.5, acid vapor, elemental carbon, and residence within 500 meters of a freeway– Analysis could not determine whether effects seen were

reversible or not

• Acute effects of PM– Delfino et al: personal PM monitoring of asthmatic children found

deficits in FEV1 for PM of multiple sources, including traffic– Koenig et al: PM associated with decreased FEV1 only in

asthmatic subset of childen– Multiple studies show association between self-reported peak

flow and PM

Cancer and Near Highway Exposures

• Clear association between PM and lung cancer in large cohort studies

• Studies that have attempted to tease out near roadway contribution to this effect problematic – Poorly controlled for ETS– Heavily trafficked roads rather than highways as source of

exposure– Modelled rather than measured air pollutants

• EPA: Diesel exhaust is likely to be carcinogenic to humans by inhalation– Vinzents et al: Bicyclists in traffic in Copenhagen had incresed

UFP exposure and oxidative damage to DNA