Research reminder: the concentration of PM2.5 is higher than that of the most polluted streets in the world

It's time to pay attention to indoor "air pollution" after being locked up at home for so long. Although we have been concerned about air pollution, most people spend more time indoors or in a closed environment in a day, especially during the epidemic.

In September 2020, an article published by PNAs introduces the changes of indoor air quality that may be caused by our behavior. The title of the article is inner works: how human biology and behavior affect indoor air quality. The Chinese title of the article is indoor work: how human biology and behavior affect indoor air quality.

Our attention to indoor air mostly focuses on the detection of chemical substances released from building materials after decoration. PAWEL wargocki, an indoor air scientist at lingerby University of technology in Denmark, explains that this type of indoor pollution has been gradually reduced due to the increasing attention paid to this aspect.

But more and more evidence shows that other daily activities can also pollute the indoor air, such as cooking, cleaning or burning aromatherapy, candles and so on. Researchers even found that people who just stay indoors can significantly change the indoor air chemical composition. Because our breathing produces heat and humidity, it is a reactive trace gas, and our skin also releases chemicals into the room.

•The impact of indoor activities

Indoor air is a complex mixture of many chemicals. Gaseous organic compounds are emitted from substances, human beings or through human behaviors, and can rapidly form into new gases or particles.

In the summer of 2018, the largest indoor air test to date was conducted in Austin: Home chem. Scientists spent a month collecting data from 18 different institutions on indoor air chemistry tests, in which volunteers only performed typical household activities in the house, such as cooking and cleaning.

The results of this study shocked us a little. There are more indoor pollution particles than we thought. Take PM2.5, which we have paid close attention to in recent years, as an example. In the cooking process, the concentration of PM2.5 in a short time exceeds the value detected on the streets of the most polluted cities in the world.

As we all know about the harm of PM2.5, because the volume of particles is small, they can enter deep into the human lungs, causing damage to the respiratory system and even cardiovascular system

Delphine farmer, an atmospheric chemist at Colorado State University and one of the leaders of homechem project, said that opening kitchen windows and ventilation vents is a simple way to reduce exposure to the air and improve indoor air quality. Of course, we should try our best to avoid unnecessary combustion.

At the same time, we may neglect the pollution caused by house cleaning. In the study, after using a commercial bleach, measurements revealed the formation of new ultrafine particles (100 nm in diameter) and elevated levels of chlorine containing gases, while indoor concentrations of hypochlorite and nitrogen trichloride were several orders of magnitude higher than those normally measured in the outdoor atmosphere.

Farmer said that in the future, the health assessment of consumer products and the warnings and suggestions on labels should not only consider the toxicity of primary compounds, but also consider the toxicity of secondary chemicals generated during or after use.

• Chemical substances released by human body

The human body itself can also release some chemicals. In a study published in 2016, Allen Goldstein's team spent two weeks measuring the volatility of carbon dioxide, ozone, and more than 200 volatile organic compounds (VOCs) in a well ventilated UC Berkeley classroom.

It was found that the content of volatile methylsiloxane in personal care products was the highest. Goldstein said that many personal care products contain methylsiloxane. The silicone oil in shampoo is polydimethylsiloxane. The more common volatile care products containing methylsiloxane include antiperspirants and deodorants.

The same trend appears in another common crowd gathering place - cinema. Jonathan Williams, an atmospheric chemist at the Max Planck Institute of chemistry in Mainz, Germany, led an experiment to determine the changes in air composition in cinemas during a movie show.

In 2013, researchers monitored changes in air composition in comedies, horror films and romantic films. The results showed that in horror tablets, isoprene reached the peak, which is a chemical released after muscle contraction, and then excreted with breathing in the human body.

In the spring of 2019, Williams, wargocki, and colleagues conducted the largest indoor study to date to measure gases released by humans: the chemicals and reactivity (ichear) study emitted by humans indoors. Each time four volunteers sat in a sealed room (usually for three hours), the researchers used a set of analytical instruments to measure the indoor air to quantify the volunteers' emissions of gases and particles over time.

In order to accurately measure the natural emissions of substances from human body, volunteers need to avoid using personal care products with fragrance or eating spicy or garlic flavored food. The clothes they wear are also those washed with low fragrance detergent by researchers.

Ichear team found that room temperature has a great influence on the release of ammonia, which is produced when bacteria break down proteins on the surface of the human body and in the intestines. Williams explained that a few degrees increase in temperature would increase ammonia emissions several times. It is found that about 90% of indoor ammonia is emitted through the skin, and the more the skin is exposed, the more ammonia is emitted.

Ichear research group is still in the process of research, and gradually turns its research target to other substances. Wargocki said they need to identify the chemicals released by the human body to better protect people who have been indoors for a long time from indoor air pollutants.

In this way, we now common indoor circulating air conditioning may be a big problem. Most U.S. homes with central air conditioning recycle nearly 100% of the air. Public and commercial buildings with mechanical ventilation usually have about 70% air recirculation.

Charles Weschler, an indoor air scientist and consultant of icear at Rutgers University, said that these practices may require us to re-examine. The proper filtering of circulating air is to use activated carbon, zeolite and silica gel to remove human pollutants to a certain extent.