Bioaerosols are airborne particles comprising of or derived from living organisms. According to Hirst (1995) “a bioaerosol is an aerosol comprising particles of biological origin or activity which may affect living things through infectivity, allergenicity, toxicity, pharmacological or other processes.”
They have a broad size spectrum (~1nm to ~102µm) with highly variable number concentrations and include viruses, bacteria, fungi, pollens, plant or animal debris, as well as fragments and products of these organisms (Grinshpun and Clark, 2005).
Bioaerosols have a range of both natural and anthropogenic sources that may lead to unique or complex assemblages at a given time and space. Health effects of bioaerosol exposure in occupational settings, for example agricultural environments, has been long established (Pepys et al. 1963) and there is growing evidence that these can range in infectivity, toxicity and allergenicity depending on variable process specific factors related to the bioaerosols (physicochemical nature) and receptors (site of deposition, immunological status) (Pearson et al. 2015; Douwes et al. 2003).
This article will briefly discuss current shifts in bioaerosols sources and the resultant regulatory needs and challenges to assess bioaerosols emissions from growing industrial sources. We will present capability requirements to advance bioaerosols risk assessment and management from industrial processes and offer a snapshot view of capabilities of a novel fluorescence based bioaerosol sensor unit with highly resolved fluorescence intensity measurements to advance bioaerosols emissions characterisation from industrial sources.
Although humans are repeatedly exposed to bioaerosols there has been growing interest in investigating environmental exposure to bioaerosols, due to changes in land use types and the resultant shifts in local to regional socio-economics over the last few decades.
The sustainable development paradigm
Since the beginning of the sustainable development paradigm in the early 1990s, there has been a growing call to shift economies from linear to circular in order to enhance resource efficiency and reduce economic and environmental challenges. Consequently, governments around the world are putting forward new polices, laws and standards to enhance resource recovery and reduce risks. For example, the EU performance target for diversion of biodegradable municipal waste from landfill requires a diversion rate of 35 percent of the 1995 levels by 2020. Currently the UK is in compliance with these targets (DEFRA, 2016) and this has resulted in substantial growth in waste management and resource recovery industry.
The recycling rates of household waste are also increasing. In the UK, during 2014 to 2015, the ‘waste from households’ recycling rate reached nearly 45% (Environment Agency, 2016). Hence, practices and technologies to manage waste have radically changed over the last 20 years and a range of established and alternative waste treatment technologies is employed in waste management infrastructure. However, concurrent with significant environmental and economic benefits, there have been concerns over emerging sources and risks associated with industrial emissions from a growing number of intensive agriculture facilities and waste management technologies – including bioaerosols emissions.
Public health risks of exposure to bioaerosols emissions from intensive agriculture and waste management industries have been investigated (Borlée et al. 2015; Douglas et al. 2016). Hence, the current focus on development of circular economies and the resultant industrial transformation to improve waste management have challenges for both operators and regulators to ensure that potential environmental and public health risks are efficiently managed alongside harnessing economic benefits. In order to issue permits for new waste treatment facilities as well as proposing and implementing bioaerosols emissions regulations from such processes, policy and decisions makers require established standards and the methods to determine compliance.
However, the evidence base to quantify both the risk of exposure to bioaerosols and the health consequences remains inadequate and moving forward with this faces methodological challenges. These are particularly diverse monitoring methods with a variety of sampling times and post sampling analysis, insufficient dose response data and diverse health end points. The existing state of knowledge on bioaerosols emissions from industrial processes is mainly in the form of short term culture based measurements and there are uncertainties in emission rates, potential consequences and probabilities of consequences.
Policy makers and regulators are faced with considerable challenges to propose proportionate risk based policies, laws and standards to allow food production and waste management infrastructures as well as ensuring risks to human and environmental health are managed. For example, in the UK a precautionary approach is adopted to planning, permitting and compliance monitoring of bioaerosols emissions from composting facilities (Environment Agency, 2010) and regulatory compliance of bioaerosols emissions is based on short term sampling and culture based methods (AfOR, 2009).
However, there are limitations associated with currently used monitoring methods that constrain taking the next step with regards informing public health risks.
The biggest constraint of currently used sampling and analysis methods is their incapability to capture spatial and temporal variability of bioaerosols emissions from industrial processes. Table 1 lists generic capability requirements categories where development of knowledge is required along with their applications and impact to advancing the risk assessment of bioaerosols emissions from industrial processes.
In recent years, a large body of research has emerged on developing and employing techniques for rapid monitoring and analysis of bioaerosols – electron microscopy; epifluorescence microscopy, laser-breakdown (LIBs), X-ray fluorescence spectroscopy, infrared (IR) absorption, Raman spectroscopy, laser-induced fluorescence (LIF), biochemical analysis (e.g. sequencing of DNA or RNA), chromatography, mass spectrometry, nuclear magnetic resonance (NMR) – mainly targeting their physical, biological or chemical properties. Significant efforts have been made to develop technical capability to monitor bioaerosols in real time.
Fluorescence spectroscopy has shown potential to quantify bioaerosols in real time and instruments based on LIF technique have recently become commercially available (Pan et al. 2015).
The findings from their recent use in different environments have shown that these can detect bioaerosols in real time (Yu et al. 2016; Ziemba et al. 2016; Saari et al. 2015; O’Connor et al. 2014). However, classification/ discrimination of bioaerosols remains the biggest challenge mainly due to broad emission bands in the existing commercially available fluorescence based detectors. Hence, development of LIF based bioaerosols detectors with narrow bands of emission spectrum to elucidate molecular origin of fluorescence is in progress.
Spectral Intensity Bioaerosol Sensor
Recently a detector based on single particle detection of fluorescence with highly resolved fluorescence intensity – Spectral Intensity Bioaerosol Sensor (SIBS) – has been developed by Droplet Measurement Technologies, USA. SIBS is an upgrade of the Wideband Integrated Bioaerosol Sensor (WIBS) and provides highly sensitive measurements of bioaerosols. It uses UV xenon flashlamp sources to excite intrinsic fluorescence in single particles in two wavebands (280nm and 370nm) followed by measurement of fluorescence intensity in 16 wavelength bands. The optical chamber in SIBS is comprised of the following components to determine particle size, shape and 16 wavelength bands of fluorescence intensity measurement, from 288 nm-735 nm:
• A continuous-wave 785nm diode laser
• A forward-scattering quadrant photomultiplier tube (PMT)
• Two pulsed xenon UV sources emitting at different wavebands (280nm and 370nm)
• A dual detector system consists of an avalanche photodiode (APD) and a 16 channel spectrometer
Hence, for each particle an estimate of particle size and shape along with 2×16 excitation-emission matrix is recorded. The particle by particle data files are stored in SIBS for offline data analysis using a data analysis toolkit to get a range of data outputs by choosing varying averaging interval, number and range of size distribution bins. A number of data outputs based on different physicochemical properties of single particles can significantly address the capability requirements to advance bioaerosols risk assessment and management for industrial processes. Table 2 presents different data outputs from SIBS and their potential application and impact to enhance our knowledge on bioaerosols emissions characterisation in the context of industrial emissions.
At present SIBS is being used, as part of a larger project focusing on detection and characterisation of inflammatory agents associated with bioaerosols emitted from biowaste and intensive agriculture, in different outdoor environments. Preliminary trials of SIBS have shown promise that fluorescence based real time measurement of bioaerosols can inform emissions characteristics from different environmental sources. For example, at a composting site, SIBS was able to capture temporal characteristics and quantify the contribution of biological particles, explaining their determinants, downwind of sources in real time. During activity (screening/turning) a substantial increase in both total and biological particles was recorded and a shift towards larger size was observed. Similarly, there were distinct differences in emission peaks and asymmetry factor during activities and background scenarios signifying the potential of real time detection and classification of bioaerosols. However, improved numerical methods are needed to analyse complex sets of data generated by SIBS. The development of new data analysis tools is ongoing.
The readiness of real time data of bioaerosols emissions characteristics from industrial processes will significantly contribute to advance the certainty and forecasting capabilities of numerical models to predict human and environmental health impacts of bioaerosols emissions from different industrial sources as well as assessing effectiveness of emissions control technologies and supporting regulatory and operational decisions. Furthermore, real time detection of bioaerosols has application in biosecurity, biosafety, environmental and public health. However, to comprehend and exploit new knowledge from highly resolved spectral signals to advance bioaerosol science novel data analysis techniques are required to deliver high impact outputs.