Road dust on an unsealed road

Road dust represents the loss of fines essential to the integrity of an unsealed road pavement, hastening the need for maintenance grading and replenishment of loosened and lost gravel. Allowing a key component of the road to be lost in this way is increasingly recognised as economically and environmentally unsustainable. Effective minimisation of road dust is integral to efficient asset management of unsealed roads. It is also a critical public health issue.

The Local Government Conference in July 2016 adopted this remit proposed by Far North District Council:

That the Crown establish and support a working party in conjunction with Local Government New Zealand, NZTA, MBIE, the Ministry of Health, Iwi and other affected parties to: (a) investigate the impact of dust on human health and the environment as it relates to our unsealed road network; and (b) develop a national strategy and policy for supporting Road Controlling Authorities to mitigate the effects of road dust on the environment and human health.

Dust has long been recognised as a problem for people living near to unsealed roads.  These problems were known to include:

  • dust landing on rooves and contaminating collected drinking water
  • dust covering gardens and household property
  • dust causing health problems for those with breathing disorders

For the road users, dust obscured the driver's vision of the road ahead, clogged filters, increased wear on moving parts, covered vehicles and penetrated passenger cabins.  Dust was regarded as a serious amenity issue for communities using or living beside unsealed roads and a potentially significant economic issue in terms of lost productivity on pastures or crops repeatedly subject to dust contamination.  Dust was not, however, traditionally recognised as a serious health hazard.

It is only since the turn of this century that the full health impacts of suspended particles (dust in the air) have begun to be recognised.  They were summarised in a 2003 study, Health Effects of PM10 in New Zealand, as: coughs, asthma symptoms, bronchitis, respiratory illness and mortality.  Prolonged exposure to particulate pollution has been found to cause lung function change, immune system responses and heart rate change.  These changes will be expressed as asthma symptoms and chronic rhinitis (hayfever).  The effects, in terms of escalating severity, will be increased visits to GPs for many individuals, hospital admission for some individuals and death for a few individuals.

Dust is defined as dry, solid particles projected into the air and settling slowly under gravity.  The particles can range in diameter from <1 micron to 100 microns.  The finer the particle, the longer it remains suspended in the air.  At 1 micron any settling due to gravity is negligible, whereas particles above 50 microns tend to settle quickly.  Similarly, particles greater than 10 microns are unlikely to reach the alveolar region of the lungs, where inhaled gases can be absorbed by the blood. Particulate matter less than 10 and 2.5 microns in diameter (PM10 and PM2.5) are key size fractions from a health perspective. This is because they are sufficiently small to penetrate the thoracic region of the lung (PM10) and have a high probability of deposition in the smaller conducting airways and alveoli (PM2.5). PM2.5 can also cross the blood-gas barrier and transport around the body causing adverse cardiovascular effects. 

In 2016 researchers in China concluded, "In summary, a wide array of experimental and epidemiological studies have unequivocally provided persuasive evidences on the negative impact of PMs on cardiovascular events and outcomes. In addition, numerous findings indicate that even a few hours to weeks of short-term exposure to PM particulates can trigger CVD-related mortality and events, especially among the susceptible individuals at great risk including the elderly or the patients with preexisting coronary artery disease" (Du et al., 2016).

Researchers in Canada have also found a significant association between mortality and the coarse fraction of particulate matter attributed to road dust (Hong et al., 2017). The study has statistically analysed daily levels of particulate matter between 10 and 2.5 microns in diameter in seven communities against daily mortality over the period 2003-2015. It found that an 8.6 μg/m3 increase in the coarse fraction particulate matter (PM10  – PM 2.5) was associated with a 3.1% increase in non-accidental mortality during the road dust season. Few other studies have considered only the coarse fraction of PM attributed to a specific source. See summary of study. See published report.

Although all individuals exposed to dust in the air repeatedly will develop some health effects, those most at risk will be the elderly, the very young, the unborn, those with pre-existing heart or lung conditions, those with asthma or diabetes, and smokers.  Many communities affected by high levels of dust pollution from traffic on unsealed roads tend to comprise high proportions of these already particularly at-risk groups.

Mitigation

It is illegal to apply used oil to roads as a dust suppressant.  A 1997 study by the Ministry for the Environment found that used engine oil contains zinc, aluminium, cadmium, chromium, lead and copper, as well as polycyclic aromatic hydrocarbons (PAH) known to be carcenogenic.  Applying used oil to suppress dust involved these potentially more harmful contaminants binding to dust particles.  When, after the lapse of sufficient time, the dust particles are transported into waterways or projected into the air, they carry these contaminants to waterways, rooves, gardens, pastures, animals and people.

The most effective method of dust suppression is to seal the road.  The cheapest suppressant is water, but its effectiveness can last from half a day to half an hour, depending on the traffic and the weather.  Proprietary dust suppressants are being introduced to the market. The keynote speaker at the 2017 Low Volume Roads Workshop, David Jones of the University of California – Davis, spoke about the benefits, tangible and intangible, of using dust suppression products as part of regular gravel road management systems to conserve fines within unsealed roads.

Using dust suppression products within normal maintenance cycles to retain fines in the pavement assists to reduce losses in the form of dust and to keep the pavement well bound over summer. Gravel replacement intervals are extended and maintenance grading frequency can be significantly reduced. The point was made that roads need to be well formed and have good drainage to benefit from using dust suppression products, and that the roads once treated still remain gravel roads. There is no benefit in an approach that applies a dust suppression product and then waits for the road to fail. Normal maintenance procedures to maintain the treated pavement need to be adopted.

The other point that was reiterated was that any chemical dust suppression should be used only in conjunction with a testing regime to evaluate the type of material to be treated. Two local companies providing chemical dust suppressant products have responded to the remarks made at the Workshop to endorse these key points. See comments from "Gravel Lock" or from "4S" on dust suppression products.

David Jones also made available the Unpaved Road Dust Control and Stabilization Treatment Selection Guide on the website of the University of California - Davis. Users must accept the disclaimer on the landing page in the website before they can fully access this Guide. 

The cost-benefit of mitigation needs to be considered carefully.  The cheapest treatment per m2, applied annually for a decade, would cost more than the likely cost of sealing the road. To assist with this process the NZ Transport Agency has released an approved methodology for assessing the merits of undertaking dust mitigation and how this fits within the investment assessment process for the Road Maintenance Activity Classes (General Circular Investment 16/04), which provides a matrix for scoring site dust risk factors and a matrix for determining the action to take, based on the results and recommendations within 2016 Research Report 590.  See NZTA General Circular Investment: 16/04.

References and resources