1. How Much Pollen Enters When You Open a Window for 1 Hour?
The Ministry of the Environment’s "Pollen Allergy Environmental Health Manual" (2022) describes an experiment conducted during the peak pollen season. In one 3LDK apartment unit, with all windows fully open for 1 hour, approximately 10,000,000 pollen grains flowed into the room [1]. Since 1 hour has 3,600 seconds, this averages out to about 2,800 grains per second. Note that this average was calculated by division and is not a figure reported directly in the experiment [1].
This number applies to one home during the busiest time. It does not mean every home receives the same amount. The large number is partly because the particles are very small. Cedar pollen has a diameter of about 25 to 30 μm (around 0.03 mm). In the manual’s electron microscope photos, they measured 28 μm and 25.4 μm [1]. The surface has small particles called orbicules attached to it [1][2]. Since 30 μm is 0.03 mm, you would need to line up about 33 grains to reach just 1 mm. These hard-to-see particles enter through windows one after another. So, is there no way to avoid opening windows?
2. How Much Does Opening a Window Only 10 cm and Using Lace Curtains Reduce Pollen?
There are days when you still want to change the air. The manual states that if you open the window to a width of about 10 cm and hang lace curtains, the pollen flowing indoors can be reduced to about one-quarter of the amount [1]. Compared to fully opening the windows, this significantly reduces the amount entering.
It is important to note that this effect comes from combining the "10 cm width" and the "lace curtains." It is not a number for just one of those factors alone. Also, while several sites like Weather News introduce the same figures, they all cite the same Ministry of the Environment manual; these were not separate experiments [4]. Therefore, you cannot simply multiply the previous 10,000,000 grains by one-quarter to say exactly 2,500,000 grains enter.
3. How Does the Amount of Pollen on Clothes Compare With Pollen Entering Through Windows?
Pollen does not only enter on the wind through windows. It also sticks to the clothes of people who have been outside and comes inside with them. A Lion Corporation research report, introduced by Weather News, states that pollen brought in on clothing can sometimes be more than 6 times the amount entering through windows during 1 hour of ventilation [3]. It is said that about 80,000 grains stuck to a wool sweater worn for 1 hour outside [3].
Be careful with the phrase "can be more than 6 times." It is not always 6 times; it is presented as a case when specific conditions are met. The figure of 80,000 is also just an example for one sweater. The 80,000 grains were on one sweater worn outside, while the 10,000,000 grains were in an apartment during 1 hour of ventilation. Because the subjects and conditions measured were different, you cannot verify a "6 times" relationship from these two numbers alone. The amount of pollen brought in on clothing cannot be known just from the window ventilation amount.
Windows are not the only entrance.
4. Which Types of Clothing and Which Body Parts Pick Up the Most Pollen?
The Ministry of the Environment manual states that wool clothing is more likely to have pollen stick to it and bring it indoors compared to cotton or synthetic fibers. When various fibers were left outside during the day for 4 hours, the number of cedar pollen grains stuck to them varied greatly depending on the type of fiber and the weave [1]. It is thought that even with the same fiber, the amount of pollen sticking changes significantly based on the weave and how it is used.
On the human body, pollen sticks easily to exposed heads, faces, and hands. Therefore, wearing a wide-brimmed hat and gloves can reduce the amount that sticks [1]. Hyogo Prefecture explains that choosing clothes and hats made of materials with fine weaves that do not easily build up static electricity is a key point for prevention [2]. When choosing a jacket, looking at the material through the lens of how easily pollen sticks can make your everyday clothes look different.
5. Where Does Pollen Settle Inside the House After It Comes In?
Pollen that enters does not stay floating in the air forever. The manual recommends cleaning because a large number of incoming pollen grains remain on floors and curtains, and washing curtains regularly [1]. As a way to prevent bringing pollen in, it also suggests not hanging laundry or futons outside [1].
Looking at it this way, pollen enters the house through several paths: windows, clothes, and hung-out laundry. It accumulates on floors and curtains. Even if you block one entrance, other paths remain.
6. How Can You Test a 10 cm Window Gap and Clothing Materials Yourself?
First, use a ruler to measure and mark what a "10 cm" width looks like on your home’s windows. It is a narrower gap than you might expect, which changes your image of the numbers.
Next, look at the tag on the clothes you are wearing now. If the material says "wool," it is a material considered likely to attract pollen according to the Ministry of the Environment manual. This gives you a clue to consider which is more likely to attract pollen: wool or cotton/synthetic fiber clothes.
If you want to read the source material, the sections on ventilation and clothing in the Ministry of the Environment’s "Pollen Allergy Environmental Health Manual" (around pages 27 and 29) are the basis. You can check the experimental conditions there.