Risk As A Metric For Air Quality Standards
Mike Holland of EMRC proposes new measurements to define an 'acceptable' level of risk from air pollutants.
In Europe this legislation takes several forms, including environmental quality standards, emission standards, fuel quality standards and fiscal incentives, such as differential fuel duties. This paper is mainly concerned with the future development of air quality standards. It is first necessary to understand that current standards do not represent no-effect thresholds, as thresholds have not been identified for the pollutants currently of most concern.
Instead, they represent levels that have been considered by experts to represent an acceptable level of risk, and can feasibly be achieved at reasonable cost.
Given that this has been done on a pollutant by pollutant basis, there is inconsistency between the standards with respect to the level of protection offered and the costs of compliance. It is also necessary to understand that current standards have been set for a series of pollutants individually, with limited attention paid to balancing risk across several pollutants taken together.
What is proposed here is a framework which aims to reduce overall risk from air pollution rather than the risks from a series of individual pollutants by prescribed amounts. This would offer greater flexibility in meeting standards, improving cost-effectiveness for achieving desired health and environmental outcomes. This, in turn, makes it possible to go further in protecting health and the environment.
When considering how future air quality standards should develop, many issues need to be considered. The approach proposed here seeks to take an integrated approach across pollutants in the following areas:
- The costs and efficiency of pollutant control options;
- The range of effects linked to each pollutant;
- The risk per unit exposure.
At its simplest, the risk model would take a set of pollutants considered to have broadly similar types of effect, for example:
Overall_risk = [ConcPM10 x CRFPM10] + [Conc03 x CRF03] + [ConcSO2 x CRFSO2] + ...
This could be expanded in several ways, the following ranked more or less in order of added complexity:
To illustrate further, Table 1 provides a worked example where the risk posed by four carcinogens at a specific location for which air quality data are as shown, is aggregated. Risk rates shown are adjusted pro-rata from lifetime risk to annual rates to facilitate comparison with cost data which are typically expressed on an annualised basis.
Table 1. Aggregation of concentration and risk data for a hypothetical location.
|Pollutant||Conc µg.m-3||Annual risk factor cancers/[million people.µg.m-3]||Concentration x risk factor cancers/million people|
From the table, it might seem at first sight appropriate to target benzene, as it is the pollutant present at highest concentration (column 2). However, the risk per unit exposure is quite low for benzene (column 3) with the result that we see in column 4 that the greatest risk at the location in question (column 4) is associated with PAHs, the pollutant present at lowest concentration.
However, most people would not be concerned with the risk associated with each pollutant individually. Instead, they would be concerned primarily with the overall risk of getting cancer. On this basis, they would focus on the total risk shown at the bottom of the table. For setting a standard they would want to know how much it would cost to reduce this risk by varying amounts. The response taken would then set about introducing the most cost-effective means for reducing total risk to an acceptable level. This freedom means that action can be targeted on the most efficient ways of reducing overall risk. In the present system action could be targeted on a pollutant simply because it is present at levels above its standard, not because it is the pollutant (or one of the pollutants) whose control offers the greatest benefit at least cost.
Whilst the proposed system is new in the context of ambient air quality standards and local air quality management, it is largely an extension of methods in cost-benefit analysis, integrated assessment modelling and regulation of industry. For example, in many ways the approach outlined here is not that different to the way that much legislation is currently developed at the European level. Under the CAFE (Clean Air For Europe) programme of European Commission DG Environment, for example, cost-benefit analysis integrates risk across a series of pollutants within a unified framework to help define optimised emission control strategies.
Another example is provided by the Environment Agency's H1 Methodology for IPPC assessment. This guides users in making decisions on what constitutes BAT for specified industrial facilities by balancing risk across a large number of burdens, including emissions to air, land and water, noise, accidents, visual impact, odour and waste.
In conclusion, the proposed system has a number of advantages over current practice for setting environmental quality standards:
The possible drawbacks of the approach are:
Our feeling is that the advantages listed here substantially outweigh the possible disadvantages, several of which could be disputed in any case. It is stressed that the paper does not dispute the value of past and current legislation that has led to substantial improvements in air quality. The system proposed here is designed to complement that work, not to replace it. A good place to start would be consolidation of the air quality standards for carcinogens.
By Mike Holland of EMRC.