At present, people’s awareness of self-protection is unprecedentedly high. In this Valentine’s Day, giving masks is more stylish than sending roses. However, with the recognition of masks, “N95” masks were not only regarded as “ancient artifacts”, but also a dozen dollars worth of goods were once fired to hundreds of yuan. When people gradually developed “maskless fear syndrome”, car companies started to make a big fuss about “air filtration” and even recently introduced the “N95-class car” concept.

The author has read a lot of “touted”, “derogated”, “questioned” and other statements on the Internet, but none of them really made clear “whether it is really effective” or even some concepts and theories are just copied, completely Without investigation and research. Therefore, with the attitude of “seeking truth and being pragmatic”, and from the perspective of your health, let everyone know the truth about this.

To understand “N95-Class Healthy Cars”, you need to understand what “N95” is.

This concept is derived from the “masks” regulations. N95 masks are NIOSH (National Institute of Occupational Safety and Health, The National Institue for Occupational Safety and Health ) certified particulate protective mask, “N” means oil-resistant, and “95” means to filter particles with a diameter of 0.3 microns with 95% efficiency, that is, (tested with 0.3 micron sodium chloride particles) , the particle concentration in the mask is lower than the external particle concentration by more than 95% (Note that 95% is not the average, but the lowest value) .

(Classification of Chinese National Standard GB 2606-2006 “Respiratory Protective Products Self-priming Filtering Anti-particulate Matter”)

The NIOSH particle respirator is divided into 9 types. In fact, N95 is the lowest standard, and higher levels are N99 and N100. (As for R and P type masks Refers to non-reusable and reusable when oily particles appear) . The US Centers for Disease Control and Prevention (CDC) recommends the use of a NIOSH-certified equivalent N95 or higher respirator to prevent inhalation of infectious particles.

(type of NIOSH certification, picture / Wikipedia)

Now that we understand the role and capabilities of N95 masks, let’s take a look at the concept of “N95 cars.”

Huacheng Certification Center, a subsidiary of China Automotive Technology and Research Center, introduced the first domestic grade N95 automotive air-conditioning filter filtration effect classification certification, namely the CN95 certification. (C stands for car) .

(CN95 certification mark)

A while ago, a car company announced that it would invest 370 million in research and development of the first domestic air-conditioning filter element with “N95 mask” as the filter standard. Subsequently, many car companies began to claim that their models have “N95 level filtering function”, and began to hype under the name of health. But after the author’s investigation and research, no matter how effective the manufacturer promotes its filtration system, or what “black technology” is used to achieve it, its core lies in one thing-HEPA filter.

HEPA (High-efficiency particulate air) is an efficiency standard for air filters. span class = “text-remarks” label = “Remarks”> (United States Department of Energy, DOE) . In other words, the HEPA filter is only a filter that meets the HEPA standard, not a specified filter. The HEPA filter is also divided into multiple grades, which are still divided according to the efficiency of filtering particles with a diameter of 0.3 microns.

(HEPA classification standard, map / Wikipedia)

This kind of filter element is actually not mysterious at all.Yes, but there may be differences in grade and quality. Note, however, that there is no such thing as a strictly defined medical-grade HEPA filter, but just a higher-level HEPA filter .

As for the vehicle filtration system, because the use environment is different from the home, it will not be as simple as a HEPA filter element. Because volatile substances and other harmful substances are also filtered, activated carbon filtration or ionizers will also be added. Equipment to improve filtration types and filtration efficiency. But in essence, to achieve virus filtration, still rely on the ability of HEPA filter.

But to understand whether the virus can be filtered, it depends on how the virus exists in the air, that is, the size of the particles with the virus.

According to the pneumonia diagnosis and treatment plan announced by the National Health and Medical Commission, the current transmission routes of new coronaviruses are: respiratory droplet transmission, contact transmission, and aerosol transmission. ( Not officially confirmed yet) . The new crown virus itself has a diameter range of about 60 to 220 nanometers (0.06 to 0.22 micrometers) and an average diameter of 100 nanometers (0.1 micrometers) , but the virus can hardly survive alone in the air.

(From left to right: particles that can also be filtered by masks 0.007 microns, neocoronavirus 0.1 microns, bacillus 0.5 microns, PM 2.5 microns, science Red blood cells 7 microns, PM 10 microns)

Flying droplets refer to daily face-to-face talking, coughing, sneezing, etc., which cause the secretions to carry the virus for transmission, but at this time, the secretion particles are large and cause rapid sinking, usually only within 1 ~ 2 Meters enter the susceptible mucosal surface. The “particle” diameter of this transmission method is generally greater than 50 microns .

(droplets when coughing)

Aerosol transmission is that droplets are evaporated to form a droplet core composed of proteins and pathogens. Due to the greatly reduced mass and volume, they can “wave in the wind” and travel a long distance. According to a paper published in the Journal of Aerosol Medicine in 2007, it was found that the “particles” in this way are diameters ranging from 0.74 to 2.12 microns .

Let’s pause here first. If you take a closer look at the above, you will find that whether it is a N95 or HEPA filter element, the filter test particle size is all 0.3 microns in diameter. Can’t I prevent the foam core? Is that a mask useless?

Yes, it’s very wrong. Most people here, even air purifier manufacturers, have made this mistake.

(an air purifier leaflet says that HEPA filters cannot filter particles smaller than 0.3 microns in diameter)

Before the epidemic, let’s think about why so many people buy “medical grade” air purifiers.

Yes, it is because of the haze formed by PM2.5. PM2.5 means fine particles with a diameter less than 2.5 microns. The aerosol formed can be suspended in the air for a long time, causing air pollution. And the air purification with HEPA filter element, verified by various authoritative laboratories, can filter most of PM2.5, isn’t this inconsistent?

The reason is that particles with a diameter of 0.3 microns are the most difficult to filter.

First of all, HEPA filter or N95, the filtering principle is not what you think “take a fishing net to fish”, but because the particle diameter is large, it is “blocked” on the Internet.

This principle is only valid for large particles. In fact, larger diameter particles are indeed “remained” in this way, but if the particle diameter is very small, , its motion trajectory may even be affected by air.The effect of the particle, thus showing a zigzag movement (Brownian motion), will eventually be adsorbed on the fiber due to Van der Waals forces and thus filtered out. (1nm = 0.001um)

According to NASA, HEPA filters are capable of filtering almost 100% of nanoparticles.

(NASA’s research report on HEPA filters)

The University of Minnesota has also conducted related experiments, and the results show that HEPA filter filters out 99.9% of particles smaller than 5 nanometers.

The same principle applies to masks. The 3M N95 mask in the test can filter particles below 7 nm with an efficiency of 96%.

Having said all this, then why both masks and HEPA filters focus on particles with a diameter of 0.3 microns? Because the particles in this diameter range are much larger than the nanoparticles, they cannot be adsorbed on the fibers according to Brownian motion, and they are not large enough, so they cannot be easily intercepted in the “gap”.

(abscissa particle diameter, ordinate filtering efficiency)

So all types of filtration products use particles with a diameter of 0.3 micron as the standard. If the interception efficiency of particles in this range is good, the filtration efficiency of other diameter particles will be better.

The same principle applies to masks with the same working principle, whether it is cotton masks, gauze masks, medical surgical masks, N95 masks, the same relationship between the filtration efficiency and particle size. (So for the new crown virus, the medical shell mask is enough, the second line in the 0.74 ~ 2.22 interval)

(Picture / “Everything About New Coronary Pneumonia” produced by the media “Paper Clip PaperCLip”)

In fact, the concept of HEPA filter elements was originally derived from the improvement of gas masks during World War II to prevent soldiers from inhaling chemical fumes; later in the 1940s, the US military further improved HEPA filter elements in order to develop radioactive substances contained in the air. It was found that particles with a diameter of 0.3 microns were the most difficult to filter out. Until the 1950s, HEPA filter elements became commercialized and continuously improved, and they are widely used in multiple industries such as aerospace, medical processing, hospitals, and nuclear industry.

To sum up, the HEPA filter itself is no different from the N95 mask, and even its actual effect (level greater than E95) is better than the N95 mask due to its volume advantage, so it can effectively block droplets with a diameter of 0.74 to 2.12 microns. Nuclear particles.

However, this does not mean that the on-vehicle filter system with HEPA filter can effectively block the new crown virus. Because wearing a mask on your face can ensure that “breathing” is in a safe environment, but for cars, “intake” does not only come from the filtering system .

The vehicle air filter system is not as simple as installing a HEPA filter element or a purifier in the car. Whether it is the use environment of the entire system or the various situations it may face, it is much more complicated than the home environment. Not only the robustness and life of the entire system must be considered, but also the air-tightness of the entire air-conditioning and filtering system itself, as well as the air-tightness of the entire vehicle.

If your car has a PM2.5 filter function, in a limited amount of space in the car, you can actually quickly improve the air quality in the car, but this does not guarantee that no new pollution will come in. Because no matter inside the air-conditioning system or in the gaps of the car door, outside air may come in directly, so the “virus protection” level of filtering cannot be guaranteed.

This is why The ambulance uses a negative pressure filtration system . Because this class of vehicles is special and costly, the strict air tightness of the entire vehicle is involved. Quickly remove potentially polluted air from outside the car,And through the negative pressure generated, the outside air is forced into the car from the designated filter system to ensure absolute medical-grade safety and quickly exclude possible contaminated air.

So, for general civilian vehicles, either the extremely high filtration system and the tightness of the entire vehicle must be ensured, or, like the “biochemical defense” mode launched by Tesla in 2016, the Model S / X is using the H13 level In addition to the HEPA filter-based filtering system, the air intake in the car is increased through the air-conditioning filtering system, so that the car always maintains a positive pressure state, which means that even if there is a possible air tightness problem, “Resist” outside air outside the car.

(Tesla Model X filtering system test results)

At present, the author has not confirmed from the data of each “N95 class car” which method is used by each company to ensure the airtightness of the system and the vehicle itself, and there is no actual test result, so it cannot be inferred Who is better.

But from a technical point of view, having HEPA filters is definitely better than ordinary air-conditioning filters. (You can change the air filter to HEPA filter by yourself)

The filter efficiency requirements of automotive air conditioning filters cover the filtering requirements of six different particle sizes from 0.3μm to 10μm. However, there is no requirement for filtering efficiency of particulate matter having a particle diameter smaller than 0.3 μm.

Secondly, the car companies that started with the earlier purification of air in the car should have relatively mature technologies and be more complete in terms of robustness, air tightness and life.

Finally, and most importantly, if you do not remember basic protective measures such as washing your hands frequently, even if the health and safety in the car is of a biochemical laboratory level, it will be useless.