Dear Editor,

We thanks Barratt and colleagues for their comments. We agree that "care should be taken to validate model estimates with empirical measurements wherever possible". Barratt and colleagues cite two stations from the European Environment Agency database as located in the Railway Ring and they report increasing NO2 concentrations from 2001 to 2005. However, one station (IT0953A) is actually located in the middle of a large park within the Railway Ring, and thus reflecting urban background concentrations, while the other station is not located in the Railway Ring, making a direct validation nearly impossible. The correct code of another traffic station located within the Railway Ring is IT0828A and the annual mean NO2 concentration went from 80 ?g/m3 in 2001 to 68 ?g/m3 in 2005, which is a clear decrease supporting our work.

Moreover, official data from the Regional Environmental Agency document that in Rome there was a decrease in nitrogen dioxide (NO2) concentrations in most of the fixed monitoring stations from 2001 to 2005.[1] Moreover, Cattani et al. have documented a decrease in both NO2 and PM concentrations, especially in sites located near traffic, over a longer period.[2]

That NOx emission standards of the different EURO vehicle classes are much smaller than initially anticipated when the policy was formulated is discovered very recently, and was not known at the time of this study. Modelling studies are hampered by assumptions about for example emission factors which may not be correct. Policy evaluation by measurements will encounter difficulties as well, particularly by other developments unrelated to the policy (for example an increase in the proportion of diesel vehicles in the London congestion charging zone). Apart from traffic other sources of air pollution on both the local and regional scale, coupled with varying meteorological conditions could all confuse air pollution trends. Therefore, multiple time windows surrounding the policy and inclusion of sites not affected by the policy should be evaluated in a proper empirical evaluation. Since this data was not readily available at the time of this study, we performed a modelling approach, reflecting real word conditions as close as possible.

Giulia Cesaroni on behalf of all the authors

References

1. http://www.arpalazio.net/main/aria/sci/basedati/chimici/chimici.php

2. Cattani G, Di Menno di Bucchianico A, Dina D, et al. Evaluation of the temporal variation of air quality in Rome, Italy, from 1999 to 2008. Ann Ist Super Sanita. 2010;46:242-53.

Conflict of Interest:

None declared

Dear Editor,

Cesaroni et al make an assessment of the health benefits of a traffic management scheme in Rome based on changes in vehicle emissions and associated chronic risk factors(1). The authors estimate that a combination of the policy intervention and unrelated fleet changes caused a 38% reduction in the annual mean exposure of NO₂ and a 29% reduction of PM10 within the 'railway ring' restricted zone between 2001 and 2005. The majority of this decrease was unrelated to the intervention, however, NO₂ reductions specifically driven by the policy, were translated to 1387 years of life gained per 100,000 residents.

We strongly believe that such statements based solely on modelled and hence theoretical decreases in pollution require validation using empirical data. Measurements from the European Environment Agency's air quality database(2) show that measured annual mean NO₂ concentrations within the 'railway ring' zone actually increased between 2001 and 2005 (80 ug/m³ to 82 ug/m³ at roadside site IT0946A and 39 ug/m³ to 41 ug/m³ at background site IT0953A). It is therefore evident that the assumptions used in the analysis did not reflect real world conditions.

Similarly, in studying the impacts of the London Congestion Charging Scheme, Kelly et al, found little evidence of a beneficial effect on monitored concentrations of NO₂ and PM10, despite a large and sustained reduction in vehicle numbers(3). This was attributed to the relatively small area of the zone and an increase in the proportion of the vehicle fleet using diesel engines. It is also now widely accepted that the Euro emission standards are not delivering the predicted reductions in NO_X(4).

While theoretical estimations of the health benefits of policy interventions are welcome, care should be taken to validate these estimates with empirical measurements wherever possible as man and machine rarely behave as predicted.

References:

1 Cesaroni G, Boogaard H, Jonkers S, Porta D, Badaloni C, Cattani G, Forastiere F, Hoek G. Health benefits of traffic-related air pollution reduction in different socioeconomic groups: the effect of low-emission zoning in Rome. Occup Environ Med. 2012;69(2):133-9.

2 http://www.eea.europa.eu/themes/air/airbase, accessed 08-mar-2012.

3 Kelly F.J., Anderson H.R., Armstrong B., Atkinson R, Barratt B., Beevers S.D, Derwent D., Green D., Mudway I., Wilkinson P., 2011. The Impact of the Congestion Charging Scheme on Air Quality in London. Research Report Number 155. Health Effects Institute, Boston, MA, USA. April 2011. Available from http://pubs.healtheffects.org/types.php?type=1.

4 Carslaw D.C., Beevers S.D., Westmoreland E., Williams W., Tate J., Murrells T., Stedman J., Li Y., Grice S., Kent A., Tsagatakis I., 2011. Trends in NOX and NO2 emissions and ambient measurements in the UK. Report for Defra, March 2011. Available from http://uk-air.defra.gov.uk/library/reports?report_id=645.

Conflict of Interest:

None declared