Air Embolism
There are two outcomes with this skill. One where air is the only means of damage which will likely end with the person stabbed suffering from an air embolism. If foreign material is injected however and the person does not suffer from an Air embolism they will likely be killed due to high-pressure injection injuries.
First things first, kids I suggest you overlook this jutsu. So far this one is the most feasible one that I believe might even be a war crime in this world if it became known. I am even going to add a war crime from my world to this design if you want to further improve this design's effectiveness.
I am going to start tame and talk about the quick death of an air embolism. Air embolism is something that has a high chance of occurring with my saturation subduing technique. It is defined as air or gas bubbles in one's circulatory system.
As I mentioned with deep sea divers this can happen when pressure is reduced on nitrogen gas in the blood but in this case, we will be using this principle in a more directed attack.
Air doesn't need to be in the body to create these bubbles for an air embolism. As long as a blood vessel is exposed to a sufficient pressure gradient in favor of the entering gas it will move from high to low pressure.
On the high end, the amount of air needed to be injected into a person's vein to be fatal is only 100 mL. If this air is injected into the arteries Termed AGE arterial gas embolism all it takes is .5 ml in the cerebral circulation and only slightly greater if introduced towards important organs like the heart.
We will be injecting wind (pressured air) into a stab. Preferably towards arteries over veins. The high pressures will expand and tear the wound making it difficult to heal and expand the wound. This also leaves you a high chance of immediately killing your enemies with the air making its way through the circulatory system hitting the brain causing stroke or heart and stopping it.
High-Pressure Injection Injuries
This is what I believe would become a war crime in this world. It feels wrong of me to write this down but I found this out in an OCHA safety video about high-pressure hydraulic leaks.
These high-pressure injection injuries are often very minor, especially in cases like I mentioned with the hydraulic oil leak which could be the size of a pinhole. As these wounds are commonly small they are often overlooked this is especially so for ninjas who have to deal with far more lethal strikes and focus on poisons.
However, these injected substances like oil often lead to severe tissue damage are amputations. This is so often the case that any injury is reported to be an injection as a potential amputee and referred to an appropriate specialist as nearly all injuries require surgery.
This is commonly an injury to the parts of our body used for fine motor control to operate the devices that can produce the PSI to produce these injuries It is known as "the most urgent of all emergencies of the hand." As most ninjas rely on the hands for Jutsu I advise being extremely careful in practicing this skill and mastering a reasonable level of medical ninjutsu in case of accidents
All it takes to breach the mundane humans' skin is about 100 PSI which is far higher than the PSI of the circulatory system and can cause death due to air embolism outright. However, the true horror of this is the chance of amputation.
The reason it is called "the most urgent of all emergencies of the hand." This is because depending on the "toxicity, density, and velocity of the injected substance, the pressure of the appliance, the anatomy and distensibility of the injection site, the possible secondary infection, and the interval between injury and treatment. Amputation rate ranges between 16% and 48% and can even reach 80% in injection injuries by paint solvents."
I couldn't tell you the medical journal this information came from as it's been over 20 years since I was interested in this topic but these numbers stuck with me.
Triangle Blade War Crime
This was usually an attachment to early ranged weapons known as firearms (this gives me ideas for future fire jutsus). These blades were so effective at killing they were banned from combat. How you might ask it was due to the difficulty of healing the wound. A triangular-shaped puncture cut in such a way that it was impossible to suture and heal. Those who were stabbed Quickly leaked blood and usually had to be amputated if they wished to live.
Adapting this shape to Jutsu can be seen as producing an injector that can be filled with an initial solution to produce high-pressure injection injuries and from there a punch Dagger to inject further pressured air. If you cannot even use this Jutsu in combat you can even seal the pressured air in a seal inside the handle of the blade.
Training
This Jutsu training is deceptively easy for the horror it can commit. All wind nature training is about how quickly one can produce wind nature chakra and use the pressure to cut durable material. All the air pressure needed to break human skin is a Mohs hardness of 2. Meaning once you start practicing with a Mohs hardness of three it no longer requires one to produce the wind before compressing it. You can add an abrasive like powder glass to increase the cutting effectiveness and wound infection.
I advise learning enough chakra flow to nature to change your chakra in a solid airtight vessel to practice. You can even save and store these highly pressurized vessels possibly filled with dirt or sand to maximize time spent practicing and preparing trump cards for combat.
Footnote
Let's see the vacuum style of wind release Jutsu. This is going to require slow power and shaping but this should be effective even underwater like with the Mantis Shrimp of my world. My worries will be shockwaves when dealing with the backlash of this skill so if you believe your control is lacking skip the next log.