There is plenty of evidence to correlate high PM concentrations with traffic, together with evidence to suggest that motor vehicle exhausts are the dominant sources of airborne PM. Additionally, there is a growing body of evidence to show that motor vehicles also contribute to another source of PM — road dust. Paul Landau investigates further.
A group of Swedish researchers1 who examined air quality in Stockholm found that road dust can not only contribute to most of the PM between 2.5 and 10 microns (PM2.5 to PM10, termed “coarser PM”), but is also correlated with increased mortality rates. Furthermore, this investigation was not isolated, but reinforces a growing body of evidence that suggests that we cannot ignore road dust. This article explains how road dust is formed, its characteristics, its impacts, and what the implications are.
Historically, whenever there are elevated concentrations of airborne PM in urban areas, vehicle exhausts have always been viewed as the prime culprit. It is easy to see why so many researchers and regulators have drawn this conclusion; vehicle exhaust gases contain PM, and there is a correlation between poor air quality and motor vehicles. While a number of studies have shown that road dust can make up a notable proportion of airborne PM, neither the level of the contribution nor its potential impact on human health were realised until the investigation in Stockholm. But the signs were there.
For example, over the border in Finland and more than 10 years earlier, a research group investigated whether there is a correlation between different types of respiratory illness in children, the sizes of PM, and their origins2. The researchers found that the situation was very complex, with different types of ailment depending on the types and sizes of PM. The team concluded that PM was highly variable, thus showing the importance of determining both the size and chemical composition of particles when considering their impacts on health.
Meanwhile, back in the 1980s and on the other side of the world, a team of researchers in Japan3 tested for the presence of road dust in the lungs of about 580 healthy people over a 5-year period. The researchers used a form of magnetic imaging and relied on the presence of small quantities of iron, which is known to characterise road dust. The period of the investigation — from 1985 to 1989 — was significant because the government at that time had a campaign to stop motorists in urban areas using studded tyres in winter. During the study, the airborne concentration of PM fell by about 40%; however, although the amount of road dust detected in people’s lungs also fell during the study period, the researchers concluded that a significant proportion of people were still breathing in road dust despite the campaign to eliminate a major source of this pollutant.
Although these studies, like many others, had alerted researchers to the potential significance of road dust as a pollutant, none had examined the effects of mortality over the long term, in a detailed and systematic way. So the Swedish researchers aimed to fill this gap.
The Stockholm investigation
Japan is similar to Scandinavia in that drivers have used studded tyres in the winter. There have been government initiatives to reduce the use of such tyres as they erode road surfaces rapidly, are noisy and result in coarse dust. Furthermore, advances in tyres mean that there are now effective winter tyres that do not need studs. At the same time, although eliminating studded tyres does improve air quality, the investigations in both nearby Finland and faraway Japan showed that tyres create road dust, even when the tyres lack studs.
The Swedish group looked at two sets of data from a nine-year period; the first dataset was PM10 and PM2.5, measured at the central station in Stockholm. The team chose this site because it was situated 25m above the ground, and therefore less likely to be affected by very local sources of pollution. This monitoring station provided a good indicator of the overall air quality in the city. Past research had shown that road dust makes up the larger sizes of particles in PM10, so the difference between PM10 and PM2.5 was important; an increase in the PM2.5 to PM10 size range could be a strong indicator of increasing proportions of road dust. On average during a year, coarser particles accounted for 42% of the total PM10, while physical and chemical analyses of the PM2.5 to PM10 range often showed a high proportion of road dust.
The second set of data was the number of daily deaths in the city. From 2000 to the end of 2008 there were about 93,400 deaths, or around 28 per day. The team then analysed the data using time-series analysis. They chose this type of statistical analysis because time-series analysis allows researchers to analyse small, acute effects in large sets of data.
In simple terms, the researchers found that when concentrations of PM2.5 to PM10 increased by 10 µg.m3, there was a 1.7% rise in the mortality rate. Obviously it would be simplistic to attribute increases in mortality rates to changes in PM concentrations — and hence increased concentrations of road dust — without also examining other factors. The researchers therefore took into account the time of year, the concentrations of other pollutants, temperature and humidity. So what did they find?
Firstly, when the likely impacts of other pollutants such as ozone, carbon monoxide and fine particulates (ie the PM2.5 fraction) were taken into account, there was not a discernible change in the increased mortality rates. When considering the seasons, the rises in mortality were higher in the winter and spring (November to May), than they were in the summer and autumn (June to October). While temperature can affect mortality, it was striking that there were more episodes of higher concentrations of coarser PM in the colder months: during November to May, the data showed that there were, on average, 148 days where the concentrations of PM2.5 to PM10 were over 20 µg.m3, compared to just four days from June to October. Notably, previous studies had shown that during the winter in Stockholm, there were higher concentrations of coarser PM due to the use of winter tyres, sand and road salt. Furthermore, these investigations had shown that road dust can contribute to 90% of the coarser PM during the winter.
Although the investigation did not examine the causes of death, the researchers concluded that road dust is a significant pollutant because many studies have shown a connection between PM2.5 to PM10 and diseases such as pulmonary inflammation and irregular heart beats. As the increases in levels of road dust were strongly correlated with higher rates of mortality, the team concluded that legislators should monitor and control road dust separately from other types of PM.
“Estimated Short-term Effects Of Coarse Particles on Daily Mortality in Stockholm, Sweden”, Environmental Health Perspectives, Meister K, et al, 2011
“Fine Particulate Air Pollution, Resuspended Road Dust and Respiratory Health Among Symptomatic Children”, European Respiratory Journal, Volume 13, Tiitanen P, et al, 1999
“Inhalation of Road Dust by Residents in Polluted Areas”, Archives of Environmental Health, Volume 47, Issue 2, Yamaya M, et al, 1992
Last reviewed 19 September 2012