# Is Fluorine more toxic than Chlorine?

Fluorine is more reactive than Chlorine. But does it mean that Fluorine can cause more damage to living tissues? If so, why wasn't Fluorine used in WW1 instead of Chlorine?

• I'd assume fluorine would be harder to manufacture, store, and handle for any side attempting to wield it in combat. Also, due to its greater reactivity, fluorine gas will tend to $\ce{2F2(g) + 2H2O(g) -> 4HF(g) + O2(g)}$ with atmospheric humidity -- unlike chlorine gas, which is less reactive and heavier and thus possibly better at staying closer to the ground and/or flowing down into an enemy's trenches. Mar 18, 2021 at 6:15
• Containing and transporting gaseous fluorine would also have been a problem for WW1 level technology. Mar 18, 2021 at 7:39
• Remember elemental Fluorine was first isolated only in 1886, after causing major discomfort to many of the leading experimentalists of the day when they did try to make it (en.wikipedia.org/wiki/Fluorine#Isolation). Its isolation was the reason Moisson won the Noble prize in 1906, for his supreme experimental technique. By 1914 industrial preparation was likely at most in its infancy. Fluorine is a pig - whatever you have heard about it, it is worse than that. Mar 18, 2021 at 8:16
• Weapons have to be practical. Poison gas overall turned out not to be very useful. But chlorine has some practicality as it is much denser than air and fills trenches and isn't so reactive it would destroy everything near its use including your own soldiers. Mar 18, 2021 at 12:07
• Fluoride ions are indeed more ‘toxic’ than chloride ions (compare the LD50 of NaF to NaCl for a rather simple example), but that’s kind of irrelevant if you just care about potential application of fluorine gas as a chemical weapon in place of chlorine gas. But then, if you want to go for impressive halogen-based chemical weapons, you should look at chlorine trifluoride instead (that stuff is so dangerous that even the Nazis decided not to use it). Mar 18, 2021 at 15:27

Fluorine is much more reactive than chlorine and would certainly cause more damage to living tissues. You can even check out https://www.youtube.com/watch?v=vtWp45Eewtw for some fun demonstrations of its oxidizing power too!

Likewise, compared to chlorine gas, I'd assume fluorine munitions would be significantly harder to manufacture, store, and handle for any side attempting to wield them in combat. (And, as Waylander mentioned in the comments, especially for "WW1-era technology" too!)

Also, due to its great reactivity, fluorine gas will readily react even with the atmospheric humidity to form hydrogen fluoride: $$\ce{2F2(g) + 2H2O(g) -> 4HF(g) + O2(g)}$$ which, while still toxic, is much lighter than chlorine gas and so may not be as good at staying "close to the ground" and/or "flowing down" into an enemy's trenches as intended. Or even worse -- potentially thus dispersing even back to your own troops and damaging them as well!

• @Joshua Chlorine trifluoride would be more practical. But think 3 times what material would be used for the canister. Mar 18, 2021 at 15:30
• @Poutnik TIL about hypervalent bonding. Fluorine is so reactive it forces chlorine to take 3 more electrons, kinda. Also, while reading, I came across "The discovery of chlorine pentafluoride rendered ClF3 obsolete as a potential rocket fuel oxidizer." So there's that. Mar 18, 2021 at 19:44
• "Fluorine is much more reactive than chlorine and would certainly cause more damage to living tissues." - Just adding that physical damage is only one of a few components of toxicity. Biochemical interactions and effects are another major component. So is radioactivity. For example, oxygen is highly reactive but the toxicity limits are extremely high, and all of the radioactive elements are toxic regardless of reactivity. Fluorine has other toxic effects, too, for example it affects thyroid hormone levels in a similar way to hypothyroidism (from unknown mechanisms). Mar 19, 2021 at 7:45
• @JasonC: Another consideration is that exposure of part of the body to a substance which destroy tissues in ways that prevent absorption into the bloodstream may be more survivable than exposure to substances that are less reactive but consequently get absorbed. Mar 19, 2021 at 16:05
• @Poutnik: Also, here's the same author's writing on chlorine trifluoride, titled "Sand Won't Save You This Time." Mar 19, 2021 at 20:39

Fluorine is in the first place much more reactive than chlorine. In contrary to chlorine, it would not damage biological tissues. It would destroy them. Pure fluorine could put the body on self-ignited fire.

Its production and manipulation would be much more expensive and difficult.

It reacts with water vapor and near anything it meets.

It would have also application disadvantage as it has just slightly greater density than air and would significantly dissipate vertically.

• Also, I would add that you can protect your own troops from chlorine reasonably well. Not so with fluorine. Mar 18, 2021 at 8:04
• Yes. It can be implied from the reactivity. Mar 18, 2021 at 8:39

From:

Section 11, toxicity:

Product Result Species Dose Exposure
fluorine LC50 Inhalation Gas. Rat 185 ppm 1 hours
chlorine LC50 Inhalation Gas. Rat 293 ppm 1 hours

So we can at least see that fluorine is more toxic to rats, for inhalation, in terms of ppm required to achieve LC50 (50% of test subjects dead) in one hour. This doesn't tell the whole story but it's relevant to gas inhalation in mammals (on the topic of WW2) and it's a good high level indicator of ultimate toxicity (regardless of specific mechanism).

The same is implied by the relative exposure limit recommendations in section 8 (I just picked comparable organizations, they're all slightly different):

Product Organization Workplace Limit
fluorine OSHA PEL 1989 (United States, 3/1989) 0.1 ppm 8 hours
chlorine OSHA PEL 1989 (United States, 3/1989) 0.5 ppm 8 hours

Although reactivity isn't necessarily an indicator of toxicity, in this case fluorine wins there, as well.

As for why they didn't use it in WW2: I don't actually have a definitive answer for this, however, according to Wikipedia:

Large-scale productions of elemental fluorine began during World War II. Germany used high-temperature electrolysis to produce tons of chlorine trifluoride, a compound planned to be used as an incendiary. The Manhattan project in the United States produced even more fluorine for use in uranium separation. Gaseous uranium hexafluoride was used to separate uranium-235, an important nuclear explosive, from the heavier uranium-238 in diffusion plants. Because uranium hexafluoride releases small quantities of corrosive fluorine, the separation plants were built with special materials.

My guess from that is two-fold:

• Production processes were relatively new, but more importantly:
• It was used in nuclear development, and at the time, resources involved in nuclear weapons research and processing were highly prioritized for that purpose. So it could have just been that they'd rather use the fluorine for weapons research.

Don't take my word for it on that one though.

• The biggest use of poison gas was WWI - so 25--30 years before large scale production of F. During WWII chemical weapons were stockpiled but not really used on the battlefield; they were of course used during the holocaust Mar 18, 2021 at 15:59

In addition to the excellent comments about handling issues (not to mention the general issues with poison gas as a weapon), production of elemental flourine remains an expensive proposition even now (most use of fluorine in industry never involves conversion to the element, but essentially movement of CaF2 to HF to various flourinated products). Conversely production of Cl2 gas was and is a massive chemical industry, with caustic soda as a byproduct. It's much easier/cheaper than making elemental fluorine.