Size classification of aerosol particles and personal consideration of masks (Column)

【Particle size range classification】

Aerosol particles with diameters of 0.01 μm or less are called nuclei mode, 0.1 μm or less are called coagulation mode, 0.1 to 1 μm are called accumulation mode, and 1 μm or more are called coarse mode.

【Particle formation and growth】

　Coagulation growth proceeds because of molecular diffusion. When air molecules collide with fine particles, the particles move at high speed. When the fine particles move at high speed, they collide with each other, which causes coagulation growth. When the particles grow to a certain size, they do not move even slightly when air molecules collide with them. Then, they do not experience coagulation growth. Particles accumulate in the size mode (0.1 to 1 μm) where no coagulation growth occurs, and this is called the accumulation mode. When particles become larger than the accumulation mode, they gravitationally fall and are removed from the atmosphere. That size (1 μm or more) is called the coarse mode.

　For particles in the coagulation mode (0.1 μm or less), the molecular diffusion motion of the particles causes them to impinge on the filter fibers during the fraction of time that the airflow passes through the filter. We consider that 100% of the particles are trapped once they collide with the massive fiber surface. Therefore, particles in the coagulation mode (0.1 μm or less) are easily collected even in coarse filters. The coarse mode (1 μm or more) is characterized by high inertia force as it moves through the airflow. On their way through the filter, they deviate from the airflow and make an inertial impact on the filter fibers. Therefore, particles in the coarse mode are easily collected even by coarse filters. Just in the meantime, particles in the cumulative region of 0.1 to 1.0 μm are less likely to be collected by air filters. Virus particles of this size are troublesome when they are suspended in the air.

【Personal thoughts on masks】

　The size of a single virus is 0.1 to 1.0 μm, so if a virus is floating by itself in the air, it is difficult to be collected with an ordinary gauze mask. However, whether the virus is floating alone in the air? is the important point. Coarse particles (droplets, etc.) that fall by gravity can be expected to be collected with high efficiency even with a coarse gauze mask. For the size of single viruses (accumulation mode), the collection efficiency of gauze masks is significantly lower.

　I am not a virologist at all, but what do you mean by a situation where a virus is spread by itself in the air? I am interested in this. If it occurs with droplets, the moisture in the fine droplets would evaporate and become smaller. The degree of evaporation depends on the relative humidity. The drier the air, the more rapidly the droplet particles will become smaller and float in the air for longer periods of time and pass through the mask more easily. "This effect may be one of the reasons why it is better to keep a room moist.

　Aerosol science has a long history of development from public health science. There was an aerosol collector (Andersen sampler) named after an old medical scientist (Mr. Andersen), and it was my favorite tool. Now, 00x masks are effective in preventing infection! Or, it is meaningless! or "It's pointless! Before being misled by such information, it is necessary to decipher who made the decision, what kind of experiments were conducted, and under what circumstances. There was a paper that showed that even with four layers of gauze fibers, more than 80% of the particles in the accumulation mode would pass through (I will note the source of the citation later).

　Incidentally, the filter mechanism for capturing particles in water is very different from the filter mechanism for air. Particles in the water do not collide with the filter fibers and become trapped. Instead, particles in the water are trapped by clogging the filter's eye holes. Particles in water filters collect particles that are larger than the filter pore diameter. This seems obvious, doesn't it? On the other hand, in an air filter, particles larger than the filter pore diameter are naturally collected, but even particles smaller than the pore diameter are collected. This is because, as mentioned earlier, particles with a small coagulation mode have a high probability of colliding with the fiber through molecular diffusion. The collection efficiency depends on the particle size, filter pore diameter, fiber layer thickness, and suction velocity, so it is difficult to determine the collection efficiency.

　My first mentor, the late Dr. NAKAE, was the president of the Air Purification Society of Japan. It is an academic society for air filters. I didn't know there was such an academic society. I was in such a laboratory from my graduation research to my master's research.