This paper is concerned with health effects of the ash aerosol formed from the co-combustion of municipal sewage sludge (a CO2 neutral, ostensibly 'green' biomass fuel) with pulverized coal. To study, and mitigate, possible lung injury caused by inhalation of these ash particles, it is useful to employ 'Health Effects Engineering', which involves collaboration between combustion researchers and toxicologists. Health Effects Engineering attempts to build connections between mechanisms that form particulates during the combustion process and mechanisms that cause these ill health effects. By employing the methods of Health Effects Engineering. one can determine not only which fuel attributes are likely to contribute to lung injury, but also how tendencies of the ash to cause lung injury can be engineered out of the combustion process. Initial results showed that inhalation of ash from the co-combustion of municipal sewage sludge (MSS) and pulverized coal caused much greater lung damage in mice, as measured by lung permeability increase, than that of coal ash. or MSS ash, alone. MSS contains substantial quantities of zinc but little sulfur, while coal contains sulfur but little zinc. Therefore, systematic experiments were conducted to determine the health effects of combustion generated zinc particles and zinc plus sulfur particles. Zinc without sulfur led to 'normal' behavior as far as lung permeability was concerned. Zinc with sulfur added led to the 'abnormal' behavior noted also in the coal+MSS experiments. Therefore, the bad actor was identified to be zinc together with sulfur, and that was why the co-combustion of coal and MSS caused greater lung injury than the combustion of either fuel alone. Health effects engineering can also be employed to diminish this health risk caused by burning fuels containing both zinc and sulfur. Injection of a kaolinite sot-bent downstream of the flame, but above the Zn dew point, can sequester the Zn, and react it to form a new species which was shown to be relatively benign. (c) 2004 Elsevier Ltd. All rights reserved.