Current gas mask filters counter current threats, but there are large gaps in knowledge about how they do so at the molecular level. Many of the filters were developed to handle a wide range of ever-changing chemical threats, and to work under a variety of different conditions.
Over the decades, a new class of chemical weapon came on to the scene. Sarin and venomous agent X, or VX, are nerve agents so named because they interfere with the nervous system's ability to communicate with muscles, including those that control breathing. The current materials used in gas mask filters provide effective protection against all of these compounds, despite the very different chemical properties of the gases.
Gas mask filters include activated carbon, a family of absorbents that traps toxins in millions of micropores. It is the same compound used to filter water and treat ingestion of poisons. The activated carbon traps the toxins, but in gas masks, it is further augmented with metal oxides, such as copper and molybdenum, to help break down the toxins. Even though the first gas mask filters were developed before these new nerve agents emerged, the current filters are effective at capturing them, and they also can break them down, but there are still questions about the chemistry of this process. The filters sometimes stop working when exposed to these organophosphorus compounds, so the chemistry of how the material is deactivated after exposure to these agents is still unknown.
Researchers are studying composite materials in respirators used by the military, police, and first responders. Studying how metal oxides interact with small organophosphates could be relevant beyond these gas masks; the work could have applications in sensing technologies. In addition, less potent forms of organophosphates are widely used as pesticides and herbicides, so the findings could help the agricultural industry and environmental scientists understand what eventually happens to these substances after they are released into the environment.
The researchers targeted two metal oxides — molybdenum oxide and copper oxide — that are key working components in gas mask filters. To simulate the small organophosphorus molecules of sarin and VX, DMMP (dimethyl methylphosphonate) was used — an established proxy for sarin with similar functional groups, but significantly lower toxicity.
The goal is to better understand the molecular interactions that occur as various gases are adsorbed by the gas mask filter materials, and the environmental conditions — air pollution, diesel fuel exhaust, water — that could alter performance and shelf life, so even better materials can be developed.
The effects of water vapor were of particular interest because of how the masks are used. The filtration mask sits in front of the mouth, so there is high humidity as one breathes into it. Water vapor seems to be neutral, or even beneficial for the performance of the materials. Water exposure activated the composite surface in a way that facilitated the binding of the DMMP molecule, lowering the energy required to break the molecule down. The effects of water, octane, and nitrogen oxides were tested, and none of these exposures decreased the ability of the metal oxides to bind to and break down DMMP molecules.