Practically every single chemical reagent, compound, and medicine carries some level of biological or environmental risk. Epinephrine is a life-saving drug, but overdoses can lead to death. Caffeine is a compound that 85% of Americans consume on a daily basis, but can lead to headaches, nausea, and diarrhea. These compounds are usually not considered entirely ‘safe’ or “unsafe”, but rather they are listed with values that are considered “acceptably safe.” Ozone is no exception to this notion. Today, we will discuss the safety aspects of ozone and demystify ozone exposure standards.
Chemical Safety Standards
Almost all industrial chemicals are assessed on the National Fire Protection Association Hazard Identification System (also known as the NFPA regulation 704). This is the same system the United States Government uses, the Hazardous Materials Identification System (HMIS). Many people have likely seen these signs in hospitals, factories, universities, or grocery store back rooms. The signs are shaped like a diamond that contains four other diamonds. Starting on the left side and moving clockwise, they are colored blue, red, yellow, and white. Here is an example:
Each of the colored squares represents a different kind of hazard: blue: health hazard; red: fire hazard; yellow: chemical reactivity; and white: specific hazard. The specific hazard can indicate a variety of things including acids, oxidizers, or radioactive materials. Each of the colored squares is graded on a scale of 0-4, with 0 being the safest and 4 being the most dangerous. Ozone has received a rating (respectively) of 3-0-2. This means that the health risks are designated “severe.” According to the HMIS these chemicals are “likely to cause major injury unless prompt action is taken and/or medical treatment is given.” But, like all chemicals, this is entirely dose and time-dependent. Many of these chemicals are used routinely, safely, and effectively on a regular basis: liquid hydrogen, calcium hypochlorite, sulfuric acid, and zinc chloride. Many people have chemicals under their kitchen sinks that carry health hazard ratings of 3 or 4.
Ozone Exposure Vocabulary
Now that we understand that the dosage amount and time of exposure are extremely important in qualifying risk, let’s take a dive into existing safety standards. Many different organizations and standards exist, so we will cover several different types. In order to understand these standards, we’ll first need to understand some of the vocabulary.
- μg/m3: This is a unit of measurement common for airborne chemicals, micrograms per cubic meter (μg/m3). This is simply a measure of density like pounds per cubic foot but with smaller units.
- PPM/PPB: These are also units of measurement, they stand for parts per million and parts per billion, respectively. PPM is the generally same functional thing as measuring in milligrams per liter (mg/L), and we will discuss how to convert between the two momentarily.
- PEL: This stands for “permissible exposure limit” and references the lower threshold that is considered safe for a specific compound.
- TWA: The time weighted average is a way to understand how much exposure an individual experienced over a given amount of time, often over the course of eight hours. For this standard, being exposed to 10 PPM (for some example chemical) for eight hours is equivalent to being exposed to 150 PPM for four hours followed by 50 PPM for four hours.
- REL: Recommended exposure level, effectively the same thing as PEL.
- TLV: The threshold limit value is similar to the REL and PEL, but the governing body that uses this, the American Conference of Governmental Industrial Hygienists (ACGIH), describes the value as “airborne concentrations of chemical substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, over a working lifetime, without adverse health effects.”
- “Ceiling”: When this term is used, it denotes the highest acceptable exposure value for the given chemical and organizational standard.
With these terms and definitions in mind, let’s discuss how to convert between PPM and μg/m3. While there is a lot of fancy math that can be discussed here, the procedure is rather simple. The conversion involves the specific mass of ozone, so this procedure will only work for ozone and cannot be generalized to convert PPM to μg/m3 for every chemical.
First, take the stated PPM value and multiply by two. Then take that value (which is now in mg/m3) and multiply by 1000 to get a value in μg/m3. In reverse, the process is the exact opposite: divide by 1000 and multiply by 2. If it is only ever necessary to convert to or from PPM, then using a factor of 2000 will suffice. It is relatively common to convert to or from PPB as well, though. In this case, 1 PPM is equivalent to 1000 PPB and 1 PPB to equal to 0.001 PPM. If it is necessary to convert from μg/m3 to PPB, it will be easiest to convert to PPM first, then convert to PPB.
Ozone Exposure Standards
Armed with the knowledge and know-how to understand and convert between various safety standards, let’s take a look at what several US and world organizations recommend.
|US EPA||150||8-hour TWA|
We can see that the recommended levels of ozone exposure, even in acute (short-term) instances, generally fall within 100-200 μg/m3.
Sleep8 and Ozone Safety
At Sleep8, we designed our products with these values in mind. In 2019, Sleep8 commissioned a study to establish the amount of ozone output by our CPAP cleaning technology. The study was conducted utilizing a standard, 9’x9’x9’ room and by running the machine for one hour. Sleep8 recommends a minimum room volume of 9’x9’x9’, and the time for a full cleaning cycle is one hour.
The study concluded that the maximum ozone concentration after an hour cleaning cycle was less than half (47%) of the maximum value listed by even the most strict regulatory agencies (WHO and FDA). Additionally, this value assumes a room with a volume of 729 ft3. If you would like to know what the concentration of ozone would be in your specific space, the calculation is simple. First, multiply the length times the width times the height (in feet) of your space. Then, divide 729 by that value and multiply by 47. The result will be the maximum concentration of ozone in μg/m3.
For example, a fairly standard master bedroom could measure 12’x12’ with 9’ ceilings. The total volume of this room is 1296 ft3. Using our formula, this would result in a max value of 27 μg/m3, only a quarter of the safety threshold.