In a study published in the journal Proceedings of the National Academy of Sciences, found that drops of saliva that flew from his mouth in loud voice, hanging in the air for 8-14 minutes. For every second of speech, the person creates an average of two and a half thousand droplets with a diameter of 10-20 micrometers. Taking into account the average viral load at COVID-19 about 40 percent of these droplets contain viral particles and can infect others patient people.
Long been known that respiratory viruses can be transmitted through particles of saliva when coughing, sneezing, or even normal speech. When we say every second in the air flying thousands of drops of saliva in size from one to 500 micrometers. These droplets may be particles of various viruses and bacteria, including the coronavirus SARS-CoV-2. However, the role of small droplets, which are formed during the speech, the transmission of the virus are poorly understood.
The settling velocity of a droplet on a surface depends on its size and the content of nonvolatile substances (e.g., proteins, sugars, DNA). Air water quickly evaporates from the drops, and she begins to fall slower: if the particle size of 50 micrometers dry to 10 micrometers, the speed of its fall will be reduced from seven to 0.35 centimeters per second. Less than a drop and the more non-volatile content, the longer it will remain in the air.
The probability of transmission through airborne droplets also affects the number of viral particles that will be in the drop. In this case, the dependence on the size of the reverse: the drop, the more virus in it and the higher the probability of infection. The average viral load of one milliliter of saliva of the patient COVID-19 is 7 million particles. The probability that a drop with a diameter of 50 micrometers contains at least one viral particle is 37 percent; for particles with a diameter of 10 micrometers, the probability decreases to 0.37 percent.
Philip Anfinrud (Philip Anfinrud) from the National institutes of health and his colleagues visualized the drops with a flat laser beam with a thickness of 1 millimeter and a height of 15 centimeters. Through a narrow slit beam fell into a dark box with a height of 60 centimeters, where a fan evenly distributes the particles in the air. One of the researchers for 25 seconds was repeated loudly in the box “stay healthy” (be healthy). This phrase was chosen because the pronunciation of the th sound particles of saliva are particularly active. 10 seconds after end of speech fan was turned off. The distribution of particles that appeared in the air, was recorded on video during the 80 minutes; the number of drops in a flat beam was calculated the total number of particles in the volume of the chamber.
In the initial time after you turn off the fan in the observation box (30 square centimeters) flew an average of nine drops. So throughout the chamber volume was more than 60 thousand particles, and the utterance of the phrase every second was thrown about two and a half thousand drops. The most vivid and, consequently, large particles are fully settled within eight minutes. Smaller drops were flying in the air for 14 minutes.