1 Introduction

Health effects assessment of exposure to particles from wood smoke

The combustion of biomasses is the oldest and, overall, most widespread energy source used in a variety of applications for heat and power production, as well as for cooking. The everyday life for a majority of the people in the world is dependent on fuels like wood, animal dung, and crop residues, as well as coal. However, according to Boman et al. (2003), only a small fraction (about 11%) of the total global energy consumption is based on biomass fuels or other combustible renewable material and waste. An increasing interest in sustainable energy production has awakened globally, and the potential for an increased use of biomass fuels is significant.

Combustion processes in general are major anthropogenic sources of many of the classical air pollutants such as carbon monoxide, nitrogen oxides, sulphur dioxides, and particulate matter, which have traditionally been associated with different adverse health effects. Residential wood combustion is often considered a major source of ambient local air pollutants, especially for particulate matter. Besides the major combustion products, carbon dioxide and water, wood smoke mainly consists of a complex mixture of particulate matter, inorganic gases, volatile organic hydrocarbons (VOC), and polycyclic aromatic compounds (PAC).

Generally, residential wood appliances are relatively old, and, compared with modern wood and pellet technology, the combustion conditions are poorly optimised. This situation often results in considerable emissions of incomplete combustion products. For example, in the United States, it has been estimated that up to 90% of the ambient wintertime PM_2.5 (mass of particles with an aerodynamic diameter of £ 2.5 µm) derive from residential wood combustion (Boman et al. 2003). In addition to the ambient exposure to wood smoke, exposure can also occur indoor as a result of direct release from heating devices or the infiltration of outdoor air pollution.

There has been a relatively extensive amount of work done concerning the adverse health effects of both individual gaseous combustion by-products (e.g. carbon monoxide, nitrogen oxides, and sulphur dioxides) as well as particulate air pollution. The adverse health effects of particulate air pollution are well documented and the impact on the respiratory function has been demonstrated in many studies. The existing knowledge is mainly related to general particulate air pollution or particles from traffic, determined as PM₁₀ or PM_2.5, and data on contributions from various sources and their individual health effects are limited.

Fewer studies have dealt with the adverse health effects of exposure to wood smoke. Since the emissions from residential wood combustion include a complex mixture of the aforementioned pollution components, it can be assumed that exposure to wood smoke is potentially harmful to human health. Today, the focus concerning emissions from residential wood combustion and its implications for air quality and human health is mainly on particulate matter.

The number of residential wood burning devices has increased dramatically in Denmark during the last decades. This has resulted in an increased exposure of the general Danish population to pollutants associated with residential wood smoke. New Danish monitoring results have shown that wood stoves can cause particle levels in residential areas that are comparable to levels at major roads. Therefore, there is an increasing concern that adverse human health effects might be associated with the increased exposure to residential wood smoke.

This project has been set up in order to review the scientific literature concerning adverse health effects of pollutants associated with residential wood smoke with the main focus on particulate matter and to quantify and evaluate, if possible, the impact on human health of the increased exposure to particles in residential wood smoke.

The report starts off with a general description of wood smoke and includes a physico-chemical characterisation of particles in wood smoke as well as in the general environment in order to evaluate whether particles in wood smoke are different from particles in the general environment (section 2).

This is followed by a summary of the available information of human exposure to particles in wood smoke with the main focus on the Danish studies and an exposure assessment is carried out for Danish conditions (section 3).

Particle deposition in the respiratory tract and clearance are addressed in section 4.

The available human and animal data on potential adverse health effects of exposure to particles from residential wood smoke (hazard assessment) are reviewed in section 5 and 6, respectively. In addition, a brief summary of adverse health effects associated with particles in the general environment is included in order to evaluate whether the adverse health effects of particles in residential wood smoke are more or less worse than those of particles in the general environment.

Finally, based on the exposure and hazard assessments, a risk characterisation is carried out for Danish conditions, if possible, i.e. if sufficient data on exposure and effects are available. In addition, the gaps of knowledge in the available data in order to perform a more refined risk characterisation are discussed.