Table 1 Current, developing and future techniques with a highlight on the study of viruses

Sampling

Sampling of bioaerosols (bacteria and fungi) is widely reviewed [5]. Bioaerosols can be collected on various media depending on the type of microbial detection, and can be collected according to their size to estimate their deposition on the respiratory system. All of these sampling techniques have pros and cons regarding the issues of size separation, sampling volume and time, biological recovery, and particle removal efficiency as well as the choice of subsequent analytical and detection methods. The sampling and quantification of viruses is less widely studied. One recent study has developed methods for airborne influenza and avian influenza virus, which is currently one of the biggest concerns of public health [11].

Detection and Identification

Detection and identification of pathogens has changed since the development of different molecular methods and innovative approaches other than culture methods [5]. The existing detection methods can be divided into two levels: generic and specific. Generic detection gives information about whether the particles are biological materials, microbes or living cells, e.g. bioluminescent measurement of ATP using continuous flow luminometer and mass-spectrometry. Specific methods such as micro-arrary and immuno-assays can tell us what kind of microbes are detected and identified. Other new techniques have been proposed for bio-detection, for instance, by characterising the size and shape of bioaerosols, pollens and fungal spores under microscope [18] and analysing fluorescence spectrum of bacteria [5].

Monitoring

It is widely recognised that background biological and chemical materials and their continuous environmental fluctuation will significantly influence monitoring. Air movement, sunlight/UV radiation, humidity, rainfall, and inversions are some of the environmental factors that need to be considered during monitoring. Another consideration is where and when to sample with regard to spatial and temporal relevance [19]. For example, the release of pathogens can cause a significant downwind hazard which requires a wide area and long period of sampling [7].

Transport/Transmission models

Epidemiology studies can link disease cases together and develop a disease transport and transmission model [8]. However, it will not always explain the mechanism. Moreover, it requires a significant number of cases in order to develop a model. The use of computational fluid dynamic (CFD) models and tracer gas simulation has demonstrated that the Severe Acute Respiratory Syndrome (SARS) virus can travel and disperse outdoors through air, and became a source of pathogens to other indoor environments [12]. A similar technique has been used in the modelling of aerosols and chemical pollutants in streets, waste treatment facilities, and other pathogen sources outdoors [7].

Biological Experimentation

Because pathogens travel in air, it is inevitable that the biological activity will be influenced by the environment. Data can be collected from field studies to determine the impact of environments on the fate and behaviour of pathogens. However, since the pathogens and environment vary and fluctuate frequently, it is not easy to build this scientific link using field data alone. Some studies have investigated the viability and environmental limits of airborne viruses and bacteria using a rotating drum and controlled climate environmental chambers [20].