Inhalation of high concentrations of airborne, so-called “low toxicity,” particles can cause adverse health effects. Epidemiological studies have repeatedly described correlations between the level of ambient particulate matter (PM) and increased morbidity and mortality in adults and children (1,2). Ambient PM has been recognized as important and even independent risk factor for both respiratory diseases like chronic obstructive pulmonary disease, asthma, and lung cancer, and also for cardiovascular disorders such as coronary artery disease, atherosclerosis, and stroke (3). The quantitative relationship between the amount of exposure (dose) to a stressor (e.g., PM) and the resulting changes in body function or health (response) is typically referred to as the dose-response relationship. The dose-response relationship-seen as causal link between a dose parameter and a toxicological effect-is often used to try to identify the driver of toxicity and to establish regulatory measures. Once the relevant dose parameter for particle toxicity is identified, dose-response relationships can be used to determine a threshold dose, which is a key issue in risk assessment and the development of (legislative) guidelines. While in numerous countries, maximum exposure levels (based on mass as dose parameter) for various types of PM are already established in workplace safety regulations (e.g., for coal or asbestos particles), similar measures have been established for ambient PM in some countries, and discussions about regulatory measures are currently intensifying for nanoparticles due to the increasing significance of nanotechnology.