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The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused millions of deaths currently. Mounting effective responses to the pandemic have become top priorities for governments around the world. Laboratory experiments have confirmed for the first time that fumigation with ozone (O3) at low O3 doses (7500–15,000 μg m−3·min) can inhibit the SARS-CoV-2 and inactivate the virus at high O3 doses (129,000 μg m−3·min). The RNA copy number of SARS-CoV-2 significantly decreased (95.9–97.7% reduction) when exposed to O3 at a low concentration of 250 μg m−3 for 30 and 60 min. A significant negative relationship between the numbers of confirmed SARS-CoV-2 cases and ambient O3 in Chinese cities indicated that the spread of SARS-CoV-2 may have been inhibited by high O3. An analysis of data from Chongqing showed that the transmission of COVID-19 increased when the O3 mixing ratios in the air were relatively low, and transmission decreased when O3 increased. Based on these findings, we also propose a new Safety O3 Emission (SOE) method to increase indoor O3 to levels (

The question of whether SARS-CoV-2 is mainly transmitted by droplets or aerosols has been highly controversial. We sought to explain this controversy through a historical analysis of transmission research in other diseases. For most of human history, the dominant paradigm was that many diseases were carried by the air, often over long distances and in a phantasmagorical way. This miasmatic paradigm was challenged in the mid to late 19th century with the rise of germ theory, and as diseases such as cholera, puerperal fever, and malaria were found to actually transmit in other ways. Motivated by his views on the importance of contact/droplet infection, and the resistance he encountered from the remaining influence of miasma theory, prominent public health official Charles Chapin in 1910 helped initiate a successful paradigm shift, deeming airborne transmission most unlikely. This new paradigm became dominant. However, the lack of understanding of aerosols led to systematic errors in the interpretation of research evidence on transmission pathways. For the next five decades, airborne transmission was considered of negligible or minor importance for all major respiratory diseases, until a demonstration of airborne transmission of tuberculosis (which had been mistakenly thought to be transmitted by droplets) in 1962. The contact/droplet paradigm remained dominant, and only a few diseases were widely accepted as airborne before COVID-19: those that were clearly transmitted to people not in the same room. The acceleration of interdisciplinary research inspired by the COVID-19 pandemic has shown that airborne transmission is a major mode of transmission for this disease, and is likely to be significant for many respiratory infectious diseases.

Breath-borne volatile organic compounds (VOCs) have been increasingly studied as non-invasive biomarkers in both medical diagnosis and environmental health research. Recently, changes in breath-borne VOC fingerprints were demonstrated in rats and humans following pollutant exposures. In this study, the eukaryotic model

Here, we investigated the use of breath-borne volatile organic compounds (VOCs) for rapid monitoring of air pollution health effects on humans. Forty-seven healthy college students were recruited, and their exhaled breath samples (

Growing air pollution, known as “April snow”, by fluffy catkins from plant flowering was frequently observed during the Spring time in China. Current efforts mainly focus on controlling catkin release, while their public health and environmental impacts are unknown. Here, using culturing, microscopic and sequence methods we found that the flying catkins carry a significant amount of bio-agents (bacteria, fungal, viruses, and pollens), e.g., up to 6.33×104 bacterial and 7.46×105 fungal cells per catkin. Abundant pollens from plants were also found riding on the flying catkins. For collected fluffy catkins, bacterial Paracoccus, Massilia, fungal Ophiocordyceps, Arthrocladiella, pollen Robina, Pickeringia, Pinus, and viral family Genomoviridae, to name a few, were found to be dominant. With infection and allergic potentials, the detected microbial structures on the fluffy catkins were different from those of local air, varying from one location to another. Being not just white pollution, flying plant fluffy catkins were discovered here also acting as a cloud of biological agents, and further actions need to be taken urgently to understand and limit their environmental and ecological consequences.

Uncertainties regarding optimized air pollution control remain as the underlying mechanisms of city-specific ambient particulate matter (PM)-induced health effects are unknown. Here, water-soluble extracts of PMs collected from four global cities via automobile air-conditioning filters were consecutively injected three times by an amount of 1, 2, and 2 mg into the blood circulation of Wistar rats after filtration by a 0.45 μm pore size membrane. Acute health effects, such as immune and inflammatory responses and hemorrhage in alveoli, were observed right after the PM extraction injection. Significant differences between cities in biomarker tumor necrosis factor-α (TNF-α) and monocyte chemoattractant protein-1 (MCP-1) levels were detected following the second and third PM injections. Rats’ inflammatory responses varied substantially with the injections of city-specific PMs. Repeated PM extract exposure rendered the rats more vulnerable to subsequent challenges, and downregulation of certain microRNAs was observed in rats. Among the studied miRNAs, miR-125b, and miR-21 were most sensitive to the PM exposure, exhibiting a negative dose–response-type relationship with a source-specific PM (oxidative potential) toxicity (r² = 0.63 and 0.57; p-values < 0.05). The results indicated that city-specific PMs could induce different health effects by selectively regulating different miRNAs, and that certain microRNAs, e.g., miR-125b and miR-21, may be externally mediated to neutralize PM-related health damages.

Airborne transmissibility of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has highlighted the urgent need for aerosol monitoring of SARS-CoV-2 to prevent sporadic outbreaks of COVID-19. The inadequate sensitivity of conventional methods and the lack of an on-site detection system limited the practical SARS-CoV-2 monitoring of aerosols in public spaces. We have developed a novel SARS-CoV-2-in-aerosol monitoring system (SIAMs) which consists of multiple portable cyclone samplers for collecting aerosols from several venues and a sensitive "sample-to-answer" microsystem employing an integrated cartridge for the analysis of SARS-CoV-2 in aerosols (iCASA) near the sampling site. By seamlessly combining viral RNA extraction based on a chitosan-modified quartz filter and "in situ" tetra-primer recombinase polymerase amplification (tpRPA) into an integrated microfluidic cartridge, iCASA can provide an ultra-high sensitivity of 20 copies/mL, which is nearly one order of magnitude greater than that of the commercial kit, and a short turnaround time of 25 min. By testing various clinical samples of nasopharyngeal swabs, saliva, and exhaled breath condensates obtained from 23 COVID-19 patients, we demonstrate that the positive rate of our system was 3.3 times higher than those of the conventional method. Combining with multiple portable cyclone samplers, we detected 52.2% (12/23) of the aerosol samples, six times higher than that of the commercial kit, collected from the isolation wards of COVID-19 patients, demonstrating the excellent performance of our system for SARS-CoV-2-in-aerosol monitoring. We envision the broad application of our microsystem in aerosol monitoring for fighting the COVID-19 pandemic.

There is great disparity in the way we think about and address different sources of environmental infection. Governments have for decades promulgated a large amount of legislation and invested heavily in food safety, sanitation, and drinking water for public health purposes. In contrast, airborne pathogens and respiratory infections, whether seasonal influenza or COVID-19, are ad-dressed fairly weakly, if at all, in terms of regulations, standards, and building design and operation, pertaining to the air we breathe. We suggest that the dramatic growth in our understanding of the mechanisms behind respiratory infection transmission should drive a paradigm shift in how we view and address the transmission of respiratory infections to protect present and future generations from unnecessary suffering and economic losses. It starts with a recognition that preventing respiratory infection, like reducing waterborne or food-borne disease, is a tractable problem. See Additional URL below for 'free to read' PDF reprint.

Rapid screening of COVID-19 is key to controlling the pandemic. However, current nucleic acid amplification involves lengthy procedures in addition to the discomfort of taking throat/nasal swabs. Here we describe potential breath-borne volatile organic compound (VOC) biomarkers together with machine learning that can be used for point-of-care screening of COVID-19. Using a commercial gas chromatograph-ion mobility spectrometer, higher levels of propanol were detected in the exhaled breath of COVID-19 patients (N= 74) and non-COVID-19 respiratory infections (RI) (N= 30) than those of non-COVID-19 controls (NC)/health care workers (HCW) (N= 87), and backgrounds (N= 87). In contrast, breath-borne acetone was found to be significantly lower for COVID-19 patients than other subjects. Twelve key endogenous VOC species using supervised machine learning models (support vector machines, gradient boosting machines (GBMs), and Random Forests) were shown to exhibit strong capabilities in discriminating COVID-19 from (HCW + NC) and RI with a precision ranging from 91% to 100%. GBM and Random Forests models can also discriminate RI patients from healthy subjects with a precision of 100%. In addition, the developed models using breath-borne VOCs could also detect a confirmed COVID-19 patient but with a false negative throat swab polymerase chain reaction test. It takes 10 min to allow an entire breath test to finish, including analysis of the 12 key VOC species. The developed technology provides a novel concept for non-invasive rapid point-of-care-test screening for COVID-19 in various scenarios.

Animal farms have been considered as the critical reservoir of antibiotic resistance genes (ARGs) and antibiotic resistant bacteria (ARB). Spread of antibiotic resistance from animal farms to the surrounding environments via aerosols has become a growing concern. Here we investigated the dispersal pattern and exposure risk of airborne ARGs (especially in zoonotic pathogens) in the environment of chicken and dairy farms. Aerosol, dust and animal feces samples were collected from the livestock houses and surrounding environments (upwind and downwind areas) for assessing ARG profiles. Antibiotic resistance phenotype and genotype of airborne Staphylococcus spp. was especially analyzed to reveal the exposure risk of airborne ARGs. Results showed that airborne ARGs were detected from upwind (50 m/100 m) and downwind (50 m/100 m/150 m) air environment, wherein at least 30% of bacterial taxa dispersed from the animal houses. Moreover, atmospheric dispersion modeling showed that airborne ARGs can disperse from the animal houses to a distance of 10 km along the wind direction. Clinically important pathogens were identified in airborne culturable bacteria. Genus of Staphylococcus, Sphingomonas and Acinetobacter were potential bacterial host of airborne ARGs. Airborne Staphylococcus spp. were isolated from the environment of chicken farm (n = 148) and dairy farm (n = 87). It is notable that all isolates from chicken-related environment were multidrug-resistance (>3 clinical-relevant antibiotics), with more than 80% of them carrying methicillin resistance gene (mecA) and associated ARGs and MGEs. Presence of numerous ARGs and diverse pathogens in dust from animal houses and the downwind residential areas indicated the accumulation of animal feces origin ARGs in bioaerosols. Employees and local residents in the chick farming environment are exposed to chicken originated ARGs and multidrug resistant Staphylococcus spp. via inhalation. This study highlights the potential exposure risks of airborne ARGs and antibiotic resistant pathogens to human health.

Coronavirus disease 2019 (COVID-19) patients exhaled millions of severe acute respiratory syndrome coronavirus 2 RNA copies per hour, which plays an important role in COVID-19 transmission. Exhaled breath had a higher positive rate (26.9%, n = 52) than surface (5.4%, n = 242) and air (3.8%, n = 26) samples.

Aerosol chemistry is often studied without considering microbial involvements. Here, we investigated time- and size-resolved bacterial aerosol dynamics in air. Under high particulate matter (PM) polluted episodes, bacterial aerosols exhibited a viability of up to 50%–70% in the 0.56–1 μm size range, at which elevated levels of SO42−, NO3− and NH4+ were concurrently observed. Engineered or acclimated for both industrial use, bacteria such as Psychrobacter spp., Massilia spp., Acinetobacter lwoffii , Exiguobacterium aurantiacum and Bacillus megaterium were shown to have experienced massive abundance shifts in polluted air on early mornings and late afternoons, on which rapid new particle formation events were widely reported. Here, Psychrobacter spp. were shown to account for >96% abundance at a corresponding PM2.5 level of 208 μg/m3. These observed bacterial aerosol changes corresponded to the PM2.5 mass peak shift from 3.2–5.6 μm to the high viability size range of 0.56–1 μm. Additionally, elevated levels of soluble Na, Ca, Mg, K, Al, Fe and P elements, required for bacterial growth, were observed to co-occur with those significant bacterial aerosol structure shifts in the air. For particular time-resolved PM2.5 pollution episodes, Acinetobacter, Psychrobacter and Massilia were shown to alternate in dominating the time-resolved aerosol community structures. The results from a HYSPLIT trajectory model simulation suggested that air mass transport played a minor role in affecting the observed bacterial aerosol structure dynamics. All the data here suggested that airborne bacteria in the size range of 0.56–1 μm could be extensively involved in aerosol chemistry in highly polluted humid air.

Antibiotic resistance genes (ARGs) have emerged as a global health concern. A large volume of work has already been devoted to ARGs in aquatic ecosystems. However, ARG dispersal patterns in air remain to be largely unknown despite of its greater role in transmission. This work aims to investigate time-resolved airborne spread of ARGs and their corresponding subtype bacterial carriers in highly polluted air. Time-resolved air samples (20 m3 each with three samples) were collected using a high volume sampler (1 m3/min) every 4 h continuously (both day and night) during low (14–93 μg/m3) and high PM2.5 (36–205 μg/m3) pollution times (over 6 days with a total of 69 air samples) in Beijing. All air samples were subjected to 16S rRNA sequence analysis for 39 ARG subtypes. Pure culturable bacterial isolates from Beijing and Shijiazhuang were Sanger sequenced for species identification and also subjected to high throughput ARG subtype detection. ARG and its subtype relative abundances in the air were observed to differ greatly (up to 3 folds for abundance) both day and night, and the blaTEM gene was found to lead the ARG abundance. For an early morning time, the multi-drug resistant NDM-1 gene was detected up to 30% of total ARG abundance in highly polluted air. Identified as a major NDM-1 and vanB gene carrier, Bacillus halotolerans were also shown to disseminate more ARG subtypes. On another front, tnpA and intI1 were shown to vary greatly in abundance, while the sul3 gene was found widespread among the culturable Bacillus isolates in the air. Principal component analysis (PCA) showed different gene co-occurrence networks for different PM2.5 pollution episodes, e.g., tnpA and intI1 for gene transfer and integration, respectively, were found more abundant for the high PM2.5 pollution episode. This study highlights a serious yet previously unidentified public health threat from time-resolved airborne spread of ARGs. Further work is urgently warranted to track the sources of ARGs for their optimized control during high PM2.5 pollution episodes. Keywords: Antibiotic resistance genes, Particulate matter, Air pollution, Time-resolved ARGs

The question of whether SARS-CoV-2 is transmitted by droplets or aerosols has been very controversial. We sought to explain this controversy through a historical analysis of transmission research in other diseases. For most of human history, many diseases were thought to transmit through the air, often over long distances and in a phantasmagorical way, and often in error (e.g. malaria, cholera). Building on the germ theory of disease developed in the mid 19th century and on the demise of miasma theory, prominent public health official Charles Chapin in 1910 urged the public health community to focus on contact and droplet infection. However, he introduced a major error in the process: that ease of infection in close proximity is associated exclusively with large “sprayborne” droplets that fall to the ground quickly, and he deemed airborne transmission as very unlikely. This new paradigm became dominant, leading to systematic errors in the interpretation of research evidence on transmission. For the next five decades, no disease was accepted by the general medical and infection control communities as airborne, until tuberculosis (which had been misclassified as droplet) in 1962. Chapin’s paradigm remained dominant and only a few diseases were widely accepted as transmitted by aerosols before COVID-19: those that were clearly transmitted over long distances or time scales. Resistance to the idea of airborne spread of a respiratory infection is not new. In fact, it has occurred repeatedly over much of the last century and greatly hampered understanding of how diseases transmit.

Interaction of water with fine particulate matter leads to the formation of reactive species (RS) that may influence the aging, properties, and health effects of atmospheric aerosols. In this study, we explore the RS yields of fine PM from remote forest (Hyytiälä, Finland) and polluted urban air (Mainz, Germany and Beijing, China) and relate these yields to different chemical constituents and reaction mechanisms. Ultrahigh-resolution mass spectrometry was used to characterize organic aerosol composition, electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique was used to determine the concentrations •OH, O2•−, and carbon- or oxygen-centered organic radicals, and a fluorometric assay was used to quantify H2O2 concentration. The mass-specific yields of radicals were lower for sampling sites with higher concentration of ambient PM2.5 (particles with a diameter 2O2 yields exhibited no clear trend. The abundances of water-soluble transition metals and aromatics in ambient PM2.5 were positively correlated with the relative fraction of •OH to the totally detected radicals, but negatively correlated with the relative fraction of carbon-centered radicals. Moreover, we found that the relative fractions of different types of radicals formed by ambient PM2.5 were comparable to the surrogate mixtures comprising transition metals, organic hydroperoxide, H2O2, and humic or fulvic acids. Therein humic and fulvic acids exhibited strong radical scavenging effect to substantially decrease the radical yield of mixtures comprising cumene hydroperoxide and Fe2+. The interplay of transition metals (e.g., iron), highly oxidized compounds (e.g., organic hydroperoxides), and complexing agents (e.g., humic or fulvic acids), leads to non-linear concentration dependencies of production and yields of different types of RS. Our findings show that how the composition of PM2.5 influences the amount and nature of RS produced upon interaction with water, which may explain differences in the chemical reactivity and health effects of particulate matter in clean and polluted air.

During the rapid rise in COVID-19 illnesses and deaths globally, and notwithstanding recommended precautions, questions are voiced about routes of transmission for this pandemic disease. Inhaling small airborne droplets is probable as a third route of infection, in addition to more widely recognized transmission via larger respiratory droplets and direct contact with infected people or contaminated surfaces. While uncertainties remain regarding the relative contributions of the different transmission pathways, we argue that existing evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit infection risk indoors. Appropriate building engineering controls include sufficient and effective ventilation, possibly enhanced by particle filtration and air disinfection, avoiding air recirculation and avoiding overcrowding. Often, such measures can be easily implemented and without much cost, but if only they are recognised as significant in contributing to infection control goals. We believe that the use of engineering controls in public buildings, including hospitals, shops, offices, schools, kindergartens, libraries, restaurants, cruise ships, elevators, conference rooms or public transport, in parallel with effective application of other controls (including isolation and quarantine, social distancing and hand hygiene), would be an additional important measure globally to reduce the likelihood of transmission and thereby protect healthcare workers, patients and the general public.

Rapid diagnosis of COVID-19 is key to controlling the pandemic. Here we report the potential breath-borne volatile organic compound (VOC) biomarkers for COVID-19. Higher levels of ethyl butanoate were detected in exhaled breath of COVID-19 patients (N=10) than healthy controls/health care workers (N=21), lung cancer (LC) patients (N=7) and backgrounds. In contrast, breath-borne butyraldehyde and isopropanol (an efficient SARS-CoV-2 inactivation agent) were significantly higher for non-COVID-19 respiratory infections (URTI) (N=22) than COVID-19, HC, LC patients and backgrounds. Breath-borne isopropanol emission from COVID-19 patients varied greatly up to ∼100-fold difference. COVID-19 patients had lower acetone levels than other subjects, except LC patients. The monitoring of ethyl butanoate, butyraldehyde and isopropanol could lend considerable support in rapidly screening COVID-19; and alerting the presence of COVID-19 patient in particular environments.One Sentence SummaryCOVID-19 patients emit distinctive VOC profiles

Despite notable efforts in airborne SARS-CoV-2 detection, no clear evidence has emerged to show how SARS-CoV-2 is emitted into the environments. Here, 35 COVID-19 subjects were recruited; exhaled breath condensate (EBC), air samples and surface swabs were collected and analyzed for SARS-CoV-2 using reverse transcription-polymerase chain reaction (RT-PCR). EBC samples had the highest positive rate (16.7%, n = 30), followed by surface swabs(5.4%, n = 242), and air samples (3.8%, n = 26). COVID-19 patients were shown to exhale SARSCoV-2 into the air at an estimated rate of 103-105 RNA copies/min; while toilet and floor surfaces represented two important SARS-CoV-2 reservoirs. Our results imply that airborne transmission of SARS-CoV-2 plays a major role in COVID-19 spread, especially during the early stages of the disease.One Sentence SummaryCOVID-19 patient exhales millions of SARS-CoV-2 particles per hour

The COVID-19 pandemic has brought an unprecedented crisis to the global health sector1. When recovering COVID-19 patients are discharged in accordance with throat or nasal swab protocols using reverse transcription polymerase chain reaction (RT-PCR), the potential risk of re-introducing the infection source to humans and the environment must be resolved 2,3,4. Here we show that 20% of COVID-19 patients, who were ready for a hospital discharge based on current guidelines, had SARS-CoV-2 in their exhaled breath (~105 RNA copies/m3). They were estimated to emit about 1400 RNA copies into the air per minute. Although fewer surface swabs (1.3%, N=318) tested positive, medical equipment frequently contacted by healthcare workers and the work shift floor were contaminated by SARS-CoV-2 in four hospitals in Wuhan. All air samples (N=44) appeared negative likely due to the dilution or inactivation through natural ventilation (1.6-3.3 m/s) and applied disinfection. Despite the low risk of cross environmental contamination in the studied hospitals, there is a critical need for strengthening the hospital discharge standards in preventing re-emergence of COVID-19 spread.

The COVID-19 pandemic is creating a havoc situation across the globe that modern society has ever seen. Despite of their paramount importance, the transmission routes of SARS-Cov-2 still remain debated among various sectors. Evidences compiled here strongly suggest that the COVID-19 could be transmitted via air in inadequately ventilated environments that are housing the infected by SARS-Cov-2. Existing experimental data showed that coronavirus survival was negatively impacted by ozone, high temperature and low humidity. Here, regression analysis showed that the spread of SARS-Cov-2 was reduced by increasing ambient ozone concentration level (48.83–94.67 μg/m3) (p-value = 0.039) and decreasing relative humidity (23.33–82.67%) (p-value = 0.002) and temperature (−13.17-19 °C) (p-value = 0.003) observed for Chinese cities during Jan-March 2020. Besides using these environmental implications, social distancing and wearing a mask are strongly encouraged to maximize the fight against the COVID-19 transmission. At no other time than now are the scientists in various disciplines around the world badly needed by the society to collectively confront this disastrous pandemic., Graphical abstract Unlabelled Image, Highlights • Experimental evidences compiled here strongly suggest that the COVID-19 could be transmitted via air in inadequately ventilated environments; • Statistical analysis showed COVID-19 transmission was negatively impacted by higher ozone level and lower relative humidity • In addition to social distancing and PPE protection, environmental parameter implications could be also utilized to help stop the pandemic

Breathing air is a fundamental human need, yet its safety, when challenged by various harmful or lethal substances, is often not properly guarded. For example, air toxicity is currently monitored only for single or limited number of known toxicants, thus failing to fully warn against possible hazardous air. Here, we discovered that within minutes living rats emitted distinctive profiles of volatile organic compounds (VOCs) via breath when exposed to various airborne toxicants such as endotoxin, O3, ricin, and CO2. Compared to background indoor air, when exposed to ricin or endotoxin aerosols breath-borne VOC levels, especially that of carbon disulfide, were shown to decrease; while their elevated levels were observed for O3 and CO2 exposures. A clear contrast in breath-borne VOCs profiles of rats between different toxicant exposures was observed with a statistical significance. Differences in MicroRNA regulations such as miR-33, miR-146a and miR-155 from rats’ blood samples revealed different mechanisms used by the rats in combating different air toxicant challenges. Similar to dogs, rats were found here to be able to sniff against toxic air by releasing a specific breath-borne VOC profile. The discovered science opens a new arena for online monitoring air toxicity and health effects of pollutants.TOC

It is rather challenging to separate atmospheric particles from nano-to micro-metre mixed in a sample. Here, a system named iSCAPE was invented to efficiently sieve particles out from a mixture by employing an electrostatic field and a non-conductive mineral oil. Tests with atmospheric particles of different cities as well as soil and road dust samples demonstrated that the iSCAPEd particles under different operating conditions moved rapidly with different velocities and both directions. Particles of different sources such as ambient air, soil or road were shown to have different polarity-charged particle fractions, and exhibited clearly different particle electrical mobility graphs after the iSCAPE sieving from seconds to minutes. Data also revealed that after the sieving some particles were enriched at specific mobility ranges. Bacterial ATP measurements implied that the iSCAPE can be also used to efficiently separate bacteria of different sizes and charge polarity. Experimental data here suggest that the iSCAPE sieving strongly replies on the electrostatic field strength, mineral oil viscosity and the run time. In theory, the iSCAPE system can be used to extract any desired targets from a complex sample, thus opening up many outstanding opportunities for environmental, biomedical and life science fields.TOC

Haze related air pollution has become one of major environmental concerns in some Asian countries. Sulfate is a main component of ambient particulate matter (PM) in the urban environment during haze episodes. Among the pollutants, sulfur dioxide (SO2) is an important precursor of sulfate and new atmospheric particle formation. However, under different atmospheric conditions its underlying formation mechanisms are not clearly elucidated. In the current work, we collected ambient PM in two cities: Zurich (Switzerland) and Beijing (China). The PM morphology and the composition were investigated by scanning electron microscopy, energy dispersive X-Ray spectroscopy and Raman micro-spectroscopy. In addition, a series of lab controlled experiments were also performed to study the SO2 reaction with CaCO3 aerosol. We found different types of CaSO4·2H2O crystals in the Chinese urban samples, with clearly different compositions than those from Zurich. The experimental data showed that the relative humidity played a significant role on the new CaSO4·2H2O crystal formation including its size, morphology and composition. Relative humidity (RH) above 80% can significantly promote SO2 oxidation on the CaCO3 particles and form the CaSO4·2H2O crystals. In contrast, at relative humidity below 40%, only few CaCO3 particles can be converted to CaSO4 particles. The results of this study facilitate the understanding of secondary inorganic aerosol formation by the reaction of CaCO3 particles with SO2 with different RHs in different city environments, and provide useful information for air pollution control.

Bioaerosol is a concept that is used to describe all biological materials suspended in the air, including bacteria, fungi, viruses, pollen, and their derivatives such as allergens, endotoxin, mycotoxins and etc. In some studies, primary biological aerosol particle (PBAP) is also coined to refer to intact microbes in the air. Bioaerosol is a multidisciplinary research subject, involving many different fields such as microbiology, mechanical engineering, air pollution, medical science, epidemiology, immunological science, biochemistry, physics, nanotechnologies and etc. The bioaerosol field has undergone about 200 years' research history since 1833 when mold spores were first detected in the air by Charles Darwin on the Cape Verde Islands. In recent decades, there has been a research boom in bioaerosol field, thus triggering many outstanding research opportunities. Visible progress has already been made in understanding bioaerosol roles in human health, atmospheric and ecological impacts as well as their respective technologies: bioaerosol capture, monitoring and also inactivation. Most recently, researchers from different fields start to bridge together for solving bioaerosol challenges and addressing key scientific problems, e.g., bioaerosol spread, real-time detection, indoor microbes, human bioaerosol emissions, and bio-defense. Toward this effort, a “Bioaerosol Xiangshan Science Conference-the 600th” has been successfully held in the summer in Beijing, China. A total of 47 scientists and funding agency officials including leading bioaerosol experts from overseas were invited and two-day long extensive discussions on bioaerosol progress and problems were carried out. Future bioaerosol directions have been outlined by the attendees during the conference. Some of the participants have also contributed to this bioaerosol special issue. This special issue consists of a total of 20 bioaerosol articles from eight countries including one review, and contributes to the advances in bioaerosol emission, transmission, health effects, ambient bioaerosols, method development and instrumentation, and control. Through this special issue, the bioaerosol community has obtained a better understanding of bioaerosol health risks and developed the corresponding strategies to confront the threats. This special issue might serve as a starting point to not only link bioaerosol scientists from different continents, but also bring together people from various fields yet with an interest in bioaerosol to collectively advance the field further.

Bioaerosols have important impacts on many scientific fields, including atmospheric environment, ecology, climate change, food safety, anti-bioterrorism, infectious diseases, public health and environmental health. This review was carried out based on the scientific outcomes from the 600th Xiangshan Science Conference held in Beijing, for whcih 47 scientists including those overseas were invited to discuss the bioaerosol research progress. Over the past 10 years, great strides have been made in the bioaerosol field, such as the expansion of research directions, research broadness and also scientists. For example, atmospheric chemistry researchers have also started to carry out studies about bioaerosol impacts on the atmospheric environment, including those on the cloud droplets, the climate and bioaerosol dynamics during haze events. In addition, new methods and instruments for bioaerosol studies are increasingly being developed in bioaerosol monitoring and capture systems, various biosensors, microfluidics, high-throughput sequencing, and they have helped the scientific community obtain a better understanding of microbes in the air. In terms of health effects, human and indoor bioaerosol emissions have become one of the hot research areas. In addition, bioaerosols were also shown to directly affect the oxidation potential of atmospheric particles and thus on the human health, however, the relevant mechanisms remain to be very limited and could be far more complex than what we have known so far. On the other hand, respiratory infections/outbreaks, caused largely by the spread of bioaerosols, continue to be a major killer for humans, especially for younger children. Currently, although a significant progress has been made in high-throughput screening of respiratory pathogens, the airborne disease transmission mechanisms are still not well understood, including the influences on the pathogen viability and effective transmission distance. The risk of military conflicts in certain regions is increasing, which could potentially lead to possible use of biological weapons via bioaerosol for mass human destruction. In addition, the airborne transmission of antibiotic resistant genes has also received wide attention as an emerging biological pollutant. By integrating sampling, microfluidics, biosensors and electronics, the scientists have taken an important step towards real-time online bioaerosol screening and biological hazard early warning. As a special type of bioaerosol, biomarkers in exhaled breath are also increasingly being studied for early disease diagnosis and screening. At the same time, surgical and nosocomial infections caused by the spread of bioaerosols in the hospital settings are also receiving wide attention. The scientific researchers have also made significant advances in the controlling airborne biological agents, e.g., microwave irradiation, cold plasma and nanomaterial filtration. This review intends to broadly review current state-of-the-art advances in bioaerosol studies, to summarize and discuss the existing problems, correspondingly providing references for the future studies. Despite of great progress made in many aspects of bioaerosols, our understanding of its significance and relevant impacts on our surroundings and health is still quite limited. In the years to come, bioaerosol research is expected to turn a new chapter if multidisciplinary collaboration can be further strengthened.