Does closing the faceshield on your full-face or modular helmet make the air you breathe dangerous?
What could be dangerous about air, you ask? Plenty. For this discussion let's focus on oxygen (O2) and carbon dioxide (CO2). Don't worry — no sophisticated chemistry is required.
At sea level, we breathe air that is about 21 percent O2 and 0.1 percent CO2. The rest is mostly nitrogen with much lesser amounts of other gasses not really important for life. We use O2 to keep our cells alive. Therefore, when you exhale, what comes out has less O2 than the air that went in. Generally, it decreases from 21 percent to 16 percent. Without a fresh supply, you run out of O2 like miners trapped during a cave-in. The miners don't run out of air, just O2. Along the way, they also generate CO2 — like you are doing now. It keeps the plants happy.
Normally, you make CO2 as a result of your cell’s machinery. It is a metabolic byproduct and you clear it by exhaling it. But what happens if, like trapped miners, you are exhaling into a closed space with no fresh air? When CO2 builds up it can have important effects on your heart rate, blood pressure and your brain. Confusion, disorientation, agitation and sleepiness will progress to passing out if the CO2 in the air you breathe continues to rise. The effects vary with both the concentration of CO2 and the duration of exposure.
Can closing your faceshield create a dangerously closed space leading to low O2 and high CO2, making you "stupid?" There is at least one study trying to answer this question by measuring CO2 concentration in a lab using a mannequin-like headform and with riders both in the lab and in the field. I am going to discount the headform data based on methodology and focus on humans instead. In the lab and on the bike, standstill with a closed faceshield resulted in a CO2 concentration of 2 percent to 2.5 percent. That is high enough to theoretically make you feel unwell (none of the riders reported this, by the way). Riding at city speeds (maybe not Spurgeon speed) as well as at highway speed (definitely not Lemmy commuting speed) all substantially decreased CO2 to quite safe levels. Interestingly, at rest and with no moving air, CO2 at a standstill was little different with the faceshield closed or open.
I am not really surprised by their findings. Air flow is a good thing and breathing at a standstill on a hot bike when there is no wind, well, sucks. The above study is from 2005, and current helmet airflow support is better. I get lots of flow from my chin vent right into the space where they were measuring CO2. I am perplexed by one thing. Why doesn’t every recreational rider feel ill if a closed faceshield raises CO2 concentrations? And more specifically, why doesn't the competitive racer — who, by the way, is generating a huge amount of CO2 behind a closed faceshield — simply keel over from CO2 intoxication and crash? They did not answer that one. No, I don't have a definitive answer, either.
But I have a few guesses that are probably better than a WAG. First, the lab and controlled circumstances are really never like the real thing. Just look at the medical data on promising drugs that work in a highly controlled test group but not in the wild. A rising CO2 concentration triggers a powerful fight-or-flight response, leading you to take a much bigger breath as well as more breaths per minute. Since none of these helmets are air-tight, you will suck in more gas that is not CO2-rich. This will restore your blood CO2 and you to normal — whatever that is for you!
What about the racer who may be sustaining a heart rate of 180 beats per minute and producing a lot of CO2 behind his or her faceshield? The study found that speeds of just 50 kph (31 mph) were enough to reduce CO2 levels to about what you'd experience without a helmet at a standstill, and racers don't spend much time below 50 kph. On some tracks, top pros are surpassing 300 kph, so all that CO2 they are exhaling gets blown away.
Therefore, it really is OK to close your faceshield, even for long periods of time. If it weren’t, we should have herds of touring riders passing out from CO2 levels. We don’t.
Sorry, you can't use a fear of CO2 intoxication as an excuse to ride with your faceshield open, wear a three-quarter or half helmet, or wear no helmet at all. If you do something stupid, it probably won't work to explain to the police officer that your judgment was addled because you were breathing too much CO2.
Feel free to crack open your faceshield at a long stop light on a hot day, however. It sure feels better.