Why Merino Wool T-Shirts Are the Right Choice for the First Day of Cool Biz
In June, Cool Biz season begins in full. As more people take off their jackets and work in just a T-shirt, many business professionals face the same issue: a regular T-shirt alone does not always maintain a clean, polished impression.
Sweat stains, odor in the afternoon, chills from air conditioning after sweating, and a sloppy impression when taking off your jacket—choosing a T-shirt for Cool Biz actually comes with many hidden pitfalls. In this article, we explain the structural causes behind these problems and how the right material can solve them.
THE PROBLEM Why “Just Wearing Cool Biz” Is Not Enough
The Cool Biz period recommended by Japan’s Ministry of the Environment runs for about five months, from June to October. For businesswear, that amounts to roughly 100 days a year. What you wear during these 100 days is not merely a matter of fashion—it directly affects your everyday cleanliness and workplace impression.
However, if you simply use cotton or synthetic T-shirts from convenience stores or mass retailers for Cool Biz, four problems tend to appear at the same time: sweat stains, odor, chills from sweat, and a look that feels too casual or underdressed. These are material-related issues, and they cannot be fundamentally solved by repeatedly buying cheap T-shirts.
COOLBIZ HISTORY
Cool Biz began in 2005 as an energy-saving initiative promoted by Japan’s Ministry of the Environment. It encouraged lighter office clothing while setting indoor air conditioning to around 28°C. At first, the main idea was “no necktie, no jacket,” but today many workplaces also allow T-shirts and polo shirts. However, this does not mean that anything goes. Cool Biz means dressing lightly while maintaining cleanliness and dignity. That is why material selection has become more important than ever.
Figure 1: Four material comparisons for Cool Biz. Cotton and synthetic T-shirts tend to struggle in all four areas, while merino wool addresses them through the structure of the material itself.
THE SCIENCE The Material Takes Care of the Function. You Just Choose the Color You Like.
There are two ways to prevent sweat stains in Cool Biz dressing: either prevent sweat from remaining in the fibers, or choose a color where stains are visually less noticeable. Merino wool achieves both at the same time.
From the perspective of fiber structure, the scales on merino wool fibers absorb and release sweat at the vapor stage rather than holding it as liquid. Cotton tends to store sweat in liquid form inside the fiber, while merino continues to release moisture as vapor. As a result, the fiber surface is less likely to remain wet. This is why merino wool structurally helps prevent visible sweat stains.
In addition, while many wool T-shirts are available only in two or three colors such as black and gray, C-one offers a wide color lineup of five to six colors per style. From classic sweat-stain-resistant shades such as dark navy and black to brighter colors such as blue, lavender, and red, this variety is one of C-one’s unique strengths among merino wool T-shirts.
Sweat stains, odor, and chills from sweat are handled by the material’s function. That means the wearer no longer has to worry about performance—only choose the color they like. This is the freedom C-one brings to businesswear.
SWEAT & FABRIC SCIENCE
The white ring-like marks caused by sweat appear when salt, proteins, urea, and other components in sweat remain in the fibers and crystallize after drying. Cotton stores moisture inside the fiber, allowing these residues to sink deeper. In contrast, the scales on the surface of merino wool fibers are hydrophobic, meaning they repel water. This makes it harder for sweat components to penetrate deep into the fiber, and they can be removed more easily with light washing. This is the chemical reason behind the feeling that merino wool is easier to care for.
THE INVESTMENT “Buying Many Cheap T-Shirts” vs “Investing in One Good T-Shirt” — The Cost Reverses Over Five Years
C-one merino wool T-shirts start from ¥13,200. At first glance, many people may feel that this is expensive. However, when divided over roughly 100 Cool Biz days, the cost is about ¥150 per day. For less than the price of a cup of coffee, you secure a clean and refined impression for the entire day.
Looking at the total cost over five years, cotton T-shirts—three shirts at ¥2,000 each replaced every year—plus ironing and additional laundry costs can exceed ¥70,000 in total. In contrast, carefully wearing one C-one T-shirt can significantly reduce long-term cost.
Figure 2: Estimated five-year total cost comparison. Although the initial investment is higher, the long-term cost can reverse thanks to less ironing and reduced laundry frequency.
WHY C-ONE Why “All Merino Wool Is the Same” Is Not True
At this point, you may be thinking: “I understand that merino wool is good. But isn’t it the same no matter which brand I choose?”
The answer is: no, it is not the same. For Cool Biz, one particularly important factor is fiber diameter. The threshold at which human skin tends to feel itchiness is around 22μm. If the fiber does not fall below this threshold, even merino wool may still feel irritating on sweaty summer skin, making long hours of wear uncomfortable.
C-one uses Super 100’s grade merino wool with a fiber diameter of 18.5μm. This is 3.5μm below the itchiness threshold, and that fineness is the physical reason behind the feeling of being able to wear it all day without skin discomfort. Some commercially available wool T-shirts that do not disclose fiber diameter may contain fibers over 22μm, which can lead to stuffiness or irritation in summer.
SUPER 100'S GRADE
“Super 100’s” is an international wool quality standard, given only to merino wool with a fiber diameter of 18.5μm or less. Many products simply labeled “merino wool” use fibers around 22–24μm, while Super 100’s belongs to a finer, higher-grade category. Whether you can comfortably wear a T-shirt through 100 days of Cool Biz depends on this difference in fiber diameter.
Spend 100 Days of Cool Biz in One T-Shirt You Will Not Regret.
From classic colors such as dark navy and black to blue, lavender, and red. With five to six colors per style, C-one offers an unusually rich color range for merino wool T-shirts. Let the material handle the function. You simply choose the color you like.
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