Asthmatics as a susceptible population in health risk assessment of airborne chemicals

Abstract

Asthma is a common chronic airway disease and about 235-300 million individuals of all ages are affected by the disease around the world. A precondition of asthma has been suggested to increase individual susceptibility to acute exposure from airborne irritant chemicals. In the health risk assessments of such chemicals, acute guideline values are derived to guide in preparedness and in emergency response, and to protect the general population, including susceptible subpopulations such as asthmatics. Experimental data on susceptible subpopulations are lacking for many chemicals and default inter-individual assessment factors (AFs) have been applied in the derivation of guideline values. However, the scientific basis for these default AFs in regard to asthmatics has been inadequate.

The aims of this thesis were threefold. First, to study the extent to which experimental data regarding chemicals tested on asthmatics are included in the derivation of different sets of acute to short-term guideline values, and second, to determine whether there is a general difference in the airway response between healthy and asthmatic individuals, and whether current AFs for inter-individual variability provide sufficient protection for asthmatics. Last, to gain more knowledge on the susceptibility of asthmatics by measuring the airway response to chlorine in naïve and allergen-sensitized mice.

In Paper I, the analysis of how asthmatics are considered in the derivation of Acute Exposure Guideline Levels (AEGLs) reveals that only 14 of 250 chemicals in the support documents had been tested on asthmatic subjects in experimental studies. A comparison between the AEGL support documents and nine other sets of acute to short-term guideline values shows that all of them were incomplete with respect to experimental data on asthmatics. In Paper II, experimental studies in which both healthy and asthmatic subjects were tested under the same conditions served as the basis for evaluating the lowest observed adverse effect concentrations (LOAECs) for healthy subjects and for asthmatic subjects. The ratios of these LOAECs were calculated and presented as estimated differential response factors (EDRFs). We found evidence of higher sensitivity among asthmatics (EDRF > 1) to 8 of 19 tested chemicals, and to 3 of 11 mixtures. Thereafter, concentration–response relationships confirmed the higher sensitivity of asthmatics to sulfuric acid and sulfur dioxide, and the Benchmark concentration (BMC) analysis of sulfur dioxide indicated a nine fold higher sensitivity among asthmatics. In Paper III, the consideration of asthmatics in the Derived NoEffect Levels (DNEL), developed under the European Union (EU) registration, evaluation, authorization and restriction of chemicals (REACH) legislation revealed that only 14 registered chemicals had been tested on asthmatics in experimental studies. Many of the DNELs for acute inhalation were higher than our estimated overall LOAEC or no observed adverse effect concentrations (NOAECs) in asthmatics, indicating a low or no safety margin. The experimental asthma model in Paper IV showed that exposure to 80 ppm of chlorine in naïve (but not OVA-sensitized) mice resulted in increased airway responsiveness and elevated numbers of neutrophils in the bronchoalveolar lavage fluid. Concentrationdependent reductions in respiratory frequency were seen in both groups. These results do not support an increased susceptibility to chlorine among mice with induced eosinophilic airway inflammation. Exclusion of asthmatics in the derivation of acute to short-term guideline values may interfere with trustful and efficient health-protective actions. The use of an AF of 10 when there is a lack of experimental data on asthmatics may be adequate to protect this subpopulation from the deleterious respiratory effects of airborne chemicals.