Bioaerosols are found everywhere in the indoor and the outdoor environments from either natural or industrial sources including building. Exposure to bioaerosols in the occupational environment is associated with a wide range of health effects with major public health impact, including infectious diseases, acute toxic effects, and, allergies as well as cancer.
Strategies developed for protecting indoor environments from deliberately used aerosol agents require efficient air filtration and air sterilization systems. In recent, the heat treatment of indoor air using thermal energy has been considered as a safe, effective and environmental friendly method, which does not use ozone, ions or filter media. Thermal energy has long been considered a suitable and reliable method for controlling microorganisms. However, most of these technologies were originally limited to control microorganisms in food or water. In addition, they may not be adequate for controlling bioaerosols, because the environment of airborne microorganisms with high fluidity is vastly different from the conditions of food or water. Therefore, it is necessary to find adequate and practical conditions for controlling bioaerosols.
In this study, bioaerosol control using thermal energy was studied experimentally including inactivation, aerosol characteristics, toxicity removal, etc. We used the bacteria and fungi microorganism as the representative bioaerosol and tested the physical and biological characteristics of bioaerosol in the exposure of elevated high temperature by thermal energy.
In the study of bacterial bioaerosol, we investigated the aerosol characteristics, inactivation, injury, and toxicity removal of bacterial bioaerosols (Escherichia coli and Bacillus subtilis) using thermal energy. A thermal electric heating system was developed in which bioaerosols were exposed to a various temperature environment with very short residence time. In addition, techniques for quantitatively sampling and measuring bacterial bioaerosol particles were used. The experimental results demonstrated the control method using thermal energy was adequate and reliable to control the viability and remove the endotoxin against bacterial bioaerosols. The size distributions of bacterial bioaerosols were kept constantly despite of exposure of high temperature of 700℃ because of very short residence time below about 0.3 sec and physically thermal resistance characteristic (e.g., peptidoglycan layers). The resulting temperature induced thermal stress influences the recovery and injury (metabolically and structurally) of collected bacteria and ultimately inactivated them. In addition, the experimental results also demonstrated the control method using thermal energy were adequate and reliable to remove toxicity (such as endotoxin) against bacterial bioaerosols.
In the study of fungal bioaerosol control, as the first step towards developing an efficient means of controlling fungal bioaerosols, we designed and evaluated a new fungal bioaerosol generator. The multi-orifice air jets and the rotating substrate inside the generator were used to uniformly scan a fungal culture plate and to maintain uniformity of the production rate of fungal bioaerosols. We conducted the experimental tests on Aspergillus versicolor and Cladosporium cladosporioides as representative indoor fungi. And, the effects of the air flow rate and the rotating speed of the substrate were investigated on the generation of fungal bioaerosols. The results demonstrated that the fungal bioaerosol generator can produce mono-dispersed fungal bioaerosols under various experimental conditions and that it is possible to control the rates of production of fungal bioaerosols by adjusting the flow rate through the fungal generator and the rotating speed of the substrate. The total particle number concentrations of the fungal spore bioaerosols that are generated increased as the rate of air flow increases. However, the decrease rate of the total particle number concentration of the fungal spore bioaerosol was reduced as the rotation period of the substrate was increased. In addition, comparing with full-air jet fungal bioaerosol generator of previous studies, more stable and higher bioaerosol number concentration was obtained. The method applied in the study will also be useful for measuring the particle release of field materials and for generating the fungal bioaerosols for exposure studies.
Finally, we investigated the effect of applied thermal energy on the culturability, toxicity removal (such as (1→3)-β-D-glucan), and aerosol characteristics of fungal bioaerosols (Aspergillus versicolor and Cladosporium cladosporioides). Fungal bioaerosols were exposed to a high temperature environment for a very short period of time. Thermal energy delivered through the electric heating tube system used demonstrated an inactivation performance for controlling fungal bioaerosols. In terms of the variation of aerosol size distribution, while the GMD decreased and the GSD increased on aerodynamic size distribution of fungal bioaerosol as the environment temperature increased. More than 99% of the A. versicolor and C. cladosporioides bioaerosols lost their culturability at 350℃ and 400℃, respectively. In addition, the experimental results also demonstrated the inactivation method using thermal energy were adequate and reliable to control (1→3)-β-D-glucan toxicity against fungal bioaerosols. Linear regression analysis showed the total (1→3)-β-D-glucan removal ratio was significantly correlated with the combined GMD reduce ratio of fungal particles. And, from SEM pictures, we found that their surface morphology of spores became smoother in conditions of elevated temperature.
In conclusion, the thermal energy for the characteristics control of airborne microorganisms will be useful to meet the increasing need for developing extensive control methods for airborne microorganism.
바이오에어로졸은 공기 중에 부유되어 있는 생물학적 입자로 정의되며, 대표적으로 박테리아와 곰팡이를 들 수 있다. 이러한 바이오에어로졸은 전염성 질병 및 알러지와 같은 인체에 유해한 영향을 끼치며, 이에 대한 예방 및 방지 기술이 다양한 방법으로 제시되고 있다. 본 연구는 그 중 열에너지를 이용한 바이오에어로졸의 특성 제어에 대한 실험적 연구이다. 열에너지를 이용한 방법은 오존이나 이온등의 2차 오염물질을 발생시키지 않으며, 필터를 사용하지 않기 때문에 보다 효율적인 바이오에어로졸 제어가 가능하다. 또한, 순간 고온 열처리 방법 등으로 기존의 식수와 식품, 의약품의 살균 방법으로 널리 적용되어온 방법이다.
본 연구는 열에너지를 이용한 박테리아와 곰팡이 바이오에어로졸의 특성 제어를 주 목적으로 하였으며, 이에 대해 물리, 화학적 특성 및 생물학적 특성에 대해 다양한 방법으로서 분석하였다. 첫 번째 연구로서 E. coli와 B. subtilis 박테리아 바이오에어로졸에 대한 열적 특성을 분석하였다. 상온에서 700도에 이르는 온도 범위에서 박테리아 바이오에어로졸을 노출 시켰으며, 노출 시간은 약 0.3초 미만이었다. 공기 중에서의 박테리아 바이오에어로졸의 입경 분포는 온도와 시간에 따라 거의 일정하였지만, 상대적인 생명성은 온도에 따라 감소하였으며, 이에 따른 생물학적인 상처도 (injury) 역시 증가하는 것으로 나타났다. 특히, Gram-negative 박테리아의 외벽에 대한 상처도가 증가한 것은 세포 외독소, 즉 엔도톡신의 감소에 영향을 준 것과 밀접한 관계가 있으며, 이에 따른 엔도톡신은 700도에서 약 80%까지 감소하는 것으로 나타났다. 두 번째 연구로서 곰팡이 바이오에어로졸 발생장치를 개발하였다. 기존의 곰팡이 바이오에어로졸 발생장치는 짧은 발생시간과 불균일한 입자 발생으로서, 전체적인 곰팡이 바이오에어로졸 제어 연구가 부족한 이유로 작용하였다. 본 연구에서는 회전판과 국부적인 에어젯 (air jet) 을 통하여 장 시간 동안 균일하고, 충분한 입자의 발생이 가능하도록 하였다. 세 번째 연구로서 곰팡이 바이오에어로졸 발생장치를 통하여 곰팡이 바이오에어로졸의 열적 특성에 대하여 연구하였다. 곰팡이의 경우 박테리아와 달리 같은 열적 조건에 대하여 공기역학적 입경의 감소와 기하학적 표준편차가 증가하는 특성을 나타내었다. 전자현미경으로서 이러한 특성이 곰팡이 입자 표면의 거칠기와 전체 크기가 감소하는 것을 확인하였다. 온도가 증가함에 따라 350도와 400도에서 A. versicolor와 C. Cladosporium 두 곰팡이의 생명성이 99.9% 감소하는 것을 알 수 있었으며, 곰팡이의 독성 물질인 베타글루칸 역시 온도에 따라 감소하는 것을 알 수 있었다. 특히 이러한 베타글루칸의 감소와 공기역학적 입경 분포와의 강한 상관관계를 갖는 것으로 나타났으며, 이로부터 열적 제어 시 곰팡이의 독성 물질 제거에 대한 간접적인 크기를 나타내는 척도로 사용이 가능할 것으로 보인다.
