實驗室排氣櫃的污染物洩漏評估

Abstract

實驗室的人員在實驗過程中常會在排氣櫃內操作危害物質，以避免有害物質的逸散。排氣櫃的捕集能力往往影響污染物逸散量多寡，而影響排氣櫃捕集能力的因素有氣流狀態、污染物狀態及操作狀態，許多研究顯示排氣櫃拉門高低會改變排氣櫃面速度的大小，進而影響排氣櫃的捕集效率。 本研究為了瞭解實驗室排氣櫃的面速度與污染物洩漏兩者間的關聯性，進行排氣櫃拉門開口於不同高度時的面速度量測以及一般操作人員呼吸區域的污染物洩漏濃度即時監測，以探討在正常運作排氣櫃時污染物的洩漏情形。研究將排氣櫃拉門劃分呈垂直平行的假想網格，量測不同區域與拉門高度間醋酸逸散量，並對應其面速度，觀察其相關性。在以兩種醋酸傾倒方式：1.有操作人員在固定時間增補醋酸量 2. 無人為操作而以滴管連續式補充醋酸，探討有無人員的操作狀態(即外部氣流干擾的有無)對排氣櫃捕集效率的影響。 結果發現有操作人員在固定時間增補醋酸量而在拉門高度設定在15~45 cm高時，面速度值（0.25~0.52 m/sec）符合各國家部分機構對排氣櫃建議值（0.3~0.77 m/sec），排氣櫃拉門高度降低，其面速度漸增，但是拉門開口過小（例如：開口高度為15 cm高），在拉門開口處會形成較強的紊流（0.50~0.52 m/sec）足以將櫃中的氣狀污染物向櫃門外洩漏，操作者的呼吸暴露值也隨之升高；而當拉門開口高度為30 cm高時，排氣櫃的排氣運作對操作者的呼吸暴露的控制，可達最佳成效。實驗也發現對於站立於櫃前正中間位置操作化學物品的操作員而言，其暴露濃度較站立兩側的人員為高。另外，操作人員傾倒溶液動作時，其原本的背景值提升，代表著操作員的動作可能產生一氣流，這產生的氣流能讓櫃中污染物洩漏至操作員的呼吸區域。 在無人為操作而以滴管連續式補充醋酸的情況下，在拉門高度設在 60~30 cm 高時，面速度值（0.28~0.49 m/sec）符合建議值，排氣櫃拉門高度降低，面速度增加，但拉門開口高度低至5cm時，面速度值（1.52~1.58 m/sec）；當拉門開口高度提高至15cm，面速度值（0.75~0.89 m/sec），均高於排氣櫃建議值。實驗結果發現拉門開口過小（例如：拉門開口為5cm高）操作者的呼吸暴露值最高；而當拉門開口高度為45cm高時，操作者的暴露控制，達排氣櫃排氣運作的最佳效果。站立於櫃前正中間不做任何操作的操作員，其暴露分析濃度較站立兩側的人員為低。另外，無人為操作櫃前呼吸區域測量的醋酸濃度和樣本的時間序列關係較有人為操作平穩，代表著站立於櫃前，未做任何動作的操作員，較不易產生氣流而將污染物帶出。

Laboratory personnel in the experimental process often operate hazardous substances in a fume hood in order to avoid the dispersion of harmful substances. The capturing efficiency of the hood often determines the emission rate of the pollutants. The factors that affect the capturing efficiency of the hood are airflow, pollutant and operational conditions. Many studies have shown that the sash position will change the face velocity of the hood, thereby affecting its efficacy.

This study investigates the relationship between the face velocity of the fume hood and the emission rate of pollutants. In order to examine the emission of the pollutants in the normal operation of the fume hood, we measured the face velocity of the hood when the sash was at different positions and monitored the exposure concentration of the pollutants at the operator’s breathing zone. The sash opening was mapped to a virtual grid for measuring the emission rate of acetic acid and face velocities at different positions when the sash was at different positions. The acetic acid was added in two ways, one with an operator adding acetic acid at regular intervals and the other with a dropper adding acetic acid continuously, to explore how the disturbances of external airflow caused by an operator affected the capturing efficiency of the fume hood.

With the operator filling acetic acid, we found that when the sash height was set within 15 ~ 45 cm, the face velocities (0.25 ~ 0.52 m / sec) met the national standard for the recommended fume hood face velocity (0.3 ~ 0.77 m / sec). When we lowered the sash, the face velocity increased. However, when the opening was too small (for example, with the sash height at 15cm), it generated strong turbulence (0.50 ~ 0.52 m / sec) inside the hood causing gaseous pollutants to leak out of the fume hood, which increased the pollutant exposure to the operator. When the sash height was set 30 at cm, it minimized the pollutant exposure to the operator. We also found that the operator standing in the middle position in front of the hood was exposed to higher pollutant concentration than that standing on the two sides. In addition, when the operator poured the solution, the original background value increased. This indicates that the movement of the operator may generate airflow, which may cause the leakage of air pollutants to the operator's breathing zone.

Without the operator and acetic acid was added with a dropper without the disturbances from external airflow, the face velocities (0.28 ~ 0.49 m/sec) with the sash height set at 60 ~ 30 cm met the recommended value. When we lowered the sash, the face velocity increased. But when the sash height was smaller than 5 cm, the face velocity was 1.52 ~ 1.58 m/sec. When the sash height was increased to 15 cm, the face velocity was 0.75 ~ 0.89 m/sec. Both of them were higher than the recommended value. The experiment data showed that when the sash opening was too small (for example, at 5 cm high), the operator’s exposure will be the highest. When the sash height was at 45 cm, the control of the pollutant exposure on the operator’s respiratory system was found to be optimal. In addition, when the operator stood in the middle position in front of the hood but did not operate, the exposure was less than the case when the operator stood on the two sides. Furthermore, the concentration of acetic acid was more stable when acetic acid was added with a dropper than the case when acetic acid was filled by an operator. It indicates that when the operator stands in front of the hood without any movement is less likely to generate airflow to cause the leakage of pollutants.