If you’ve read any headlines about gas stoves lately, and you’re not scared for those who use them, you’ve got a really thick skin.
I watched a YouTube video from ASAPScience this morning, and I had to do a hard stop. When the guy said that using a gas stove would produce enough NOx to exceed the EPA’s National Ambient Air Quality Standard (NAAQS), I just had to stop. The fear-mongering was too much for me. And I realized I need to address this.
Here’s the deal — no, you’re not going to exceed the NAAQS for NO2 (nitrogen dioxide) if you are a normal user. Full stop. That’s a fact. The reason for the confusion is that people do not understand how the NAAQS are set-up — the NAAQS are, in fact, very, very confusing. They even confuse toxicologists and risk assessors who don’t work on air quality. But don’t worry, down further in the post I explain the NAAQS for NO2 in a bit more detail.
Now, you might be saying — there are lots of other contaminants. And yes, there are. Did I analyze data for those? No, I didn’t. So to be clear, this discussion is focused on the safety with respect to NO2. I may write about the other contaminants at some point. But, based on what I’m seeing for NO2, and what I know of the other contaminants, I’m not too concerned, assuming you’re not cooking in a very tightly sealed box. As long as you have ventilation and air changes going on, you’re probably fine. But facts and circumstances — I’m sure there’s a house somewhere where I’d say, “yeah, no, definitely don’t use gas” — but for most people, for the average user, who is using their gas stove appropriately and has the recommended levels of ventilation, based on the available science, you’re more likely than not to be fine.
If you want to know why I say you don’t have to worry about NO2, read on. Else, USE YOUR RANGE HOOD!!!!
First We Need To Understand Gas Stoves and Energy/Heat Production
Before we can calculate NOx levels, we first need to explore the concept of energy production from gas — because that tells us how much energy is being used to cook, and that energy is what drives the cooking process.
Most of us are going to be somewhat familiar with BTUs (British Thermal Units). The BTU is the amount of energy that we will be using when cooking. Different types of cooking will require different BTUs. And different sizes of pots and pans will require different burner sizes, which also impacts the BTUs. And because Lebel et al. (2022) gave us NOx per Joule, we will need to convert our BTUs to Joules — fortunately that’s pretty straightforward.
I’m not a gas stove aficionado — I use electric. The only time I care about BTUs is when I’m buying a gas grill. So, I turned to Whirlpool to help me understand the BTUs associated with different burners and different cooking styles. According to Whirlpool, simmer burners are usually 500-2,000 BTU per hour (BTU/hr). Burners producing 2,000-10,000 BTU/hr work well for sauteing and frying. Burners producing 12,000-18,000 BTU/hr are for high-heat activities like searing or stir-frying.
I’m also not a chef. But what I do know is how not to burn my family’s dinner every night (some nights I’m off my game). Stir-frying is not something you’re going to be doing for a long period of time. You’re also not going to be searing for a long period of time.
So, let’s say you’re doing some average cooking. If you’re in my house that would be on a burner producing 9,000 BTU/hr. Let’s say you’re cooking on this burner near high (that’s where I’m getting my 9,000 BTU/hr number, btw) for 20 minutes. Since 20 minutes is 1/3 of an hour, that means that we’re using 9,000 BTU/hr * 1/3 hr = 3,000 BTU.
Each BTU = 1,055.05583 Joules (J). So 3,000 BTU is about 3,165,168 J. That’s a lot of Joules!
How Much NOx Do We Generate When Using Gas?
Brass tacks time. How much NOx am I producing with my 20 minutes of cooking that produces 3,165,168 J?
According to Dennekamp et al (2001) and Lebel et al (2022) levels of 2000 ppb NOx should be expected at their peak during cooking. NOx levels will go down rather quickly as nitrogen dioxide and nitrous oxide are absorbed on to surfaces.
The Lebel et al. (2022) paper reports that 21.7ng of NOx is generated per Joule produced from burning natural gas. So let’s do this:
(21.7 ng NOx / J) * 3165168 J = 68,684,145.6 ng NOx
Next, let’s convert these into more relatable units:
68,684,145.6 ng NOx * (1 ug/1000 ng) * (1 mg/1000 ug) = 68 mg NOx
Next, let’s figure out the amount of air in our kitchen. I’m making some health conservative assumptions, like I’m assuming you have a mostly enclosed kitchen in your house. This matters because we are assuming that the volume of air available to disperse the NOx is smaller than if you had an open concept floorplan or a loft — in those cases you have more air available for dispersal.
So let’s assume your kitchen is 10′ x 10′ with 8′ ceilings. That gives you 800 cubic feet of air volume. Now let’s translate that to cubic meters:
(800 ft^3) * ((1 m^3)/(35.315 ft^3)) = 22.65 m^3
And now we calculate how much NOx we have per cubic meter of air:
(68 mg NOx) / (22.65 m^3) = 3 mg/m^3
Okay, so now we need to convert this to parts per billion so that we can compare against the NAAQS. To do that, we’re going to use some formulas that the EPA kindly put out.
Concentration (ppb) = 24.45 * 3 mg/m3 * (1000 ug/1 mg) * (1/46 g/mol) = 1,594.6 ppb
How Does This Compare to the National Ambient Air Quality Standards for NO2?
So here’s what the NAAQS for NO2 say:
- “1-hour standard…at a level of 100 parts per billion (ppb)…based on the 98th percentile of the annual distribution of daily maximum 1-hour NO2 concentrations, averaged over 3 years.”
- “an annual standard…at a level of 53 ppb…based on annual average NO2 concentrations.”
Now, this is where a lot of people say, “Hey, this violates the NAAQS!!” And this is where I say, not so fast, it’s rather complicated.
So, let’s look at the 1-hour standard. This is based on a 3-year period. So you take all the 1-hour NO2 concentrations for 3 years, and you plot the distribution. If the 98th percentile is less than 100 ppb, then you’re in attainment — meaning you have not violated the standard.
The annual standard is also based on 1 hour measurements. For more information on how this is calculated, see this Federal Register entry. Now, go down to Appendix S — that’s where EPA explains how to calculate the annual NO2 concentrations. There you will see EPA defines “Annual Mean” as “the annual average of all of the 1-hour concentration values…”
Doing a “Real-World” Calculation/Estimate
If you’re in my house, most cooking happens for dinner. Breakfast is usually cereal, microwaved instant oatmeal, yogurt, etc, but rarely do we cook in the morning. So our NOx levels in the kitchen mostly are whatever the ambient levels are. As I’m writing this, the nearest monitor says my NO2 readings averaging around 17 ppb. So let’s just assume in general it’s around 17 ppb.
If I cook for 20 minutes, the NOx will go up to 1,594.6 ppb. According to Dennekamp et al (2001), NOx levels have a half-life of about 45 minutes. So after 135 minutes we’ve returned to near background levels.
So what this means is that for about 90 minutes we have elevated NOx levels, every day. Let’s call it 120 minutes to make the math easier. So for 2 out of 24 hrs each day we have really elevated NOx levels. That means if we tend to average 17 ppb all day, except for 2 hours, we would have an overall daily average of:
(17 * 22 + 1594.6 * 1/3 + 1594.6/2 + (1594.6/2) / 2) / 24 = 87.56 ppb
So now, some of you will say, “A ha! See we exceeded the NAAQS for NO2!” To this I say, well, not quite so fast. For starters, I’m calculating total NOx, which includes NO2 and nitrous acid and nitric acid. So my estimates are going to be a little higher than the amount of NO2, but I want to be health protective, so I’m going to say all my NOx are NO2.
But, more importantly, there’s something missing from my calculations — air changes! ASHRAE recommends “0.35 air changes per hour but not less than 15 cubic feet of air per minute (cfm) per person.” 15 ft^3/min is equal to 0.425 m^3/min. If we assume a 2,000 sq ft home, with 8′ ceilings, that’s 453 cubic meters. If the kitchen is 22.65 m^3, then it’s 5% of the air volume of the home. That means that 0.02 m^3/min of the kitchen air is cleaned on average every minute. This means that every minute 0.06mg of NO2 is eliminated from the kitchen due to air changes. That means that due simply to air exchanges, assuming no deposition (which we know deposition is happening), that the NOx due to cooking would disappear in 50 minutes. That means that the average for the day calculated above is significantly overestimating the amount of NOx present on average for each day. In other words — you’re not going to exceed the NAAQS if your home was built to ASHRAE standards.
Moreover, if you have a 600 cfm range hood, then you are cleaning 16.99 m^3/min. In other words, just by using your range hood, you are removing all of the NOx produced by cooking!
Let me say that again: By using your range hood, you are removing ALL OF THE NOX produced by cooking. ALL OF IT.
Back to Reality — Most Kitchens Aren’t Self-Enclosed Boxes
So the math that I did above mostly assumed your kitchen is a self-enclosed box. But that’s not how most kitchens in most homes are designed.
Your kitchen more than likely has airflow to other rooms in the house.
Why is this important? Because the more air that the NO2 can disperse to, the more the NO2 is being diluted. When someone cooks eggs in our house on a Saturday morning (which is a rare treat), we can smell it in every room of the house, downstairs (where the kitchen is) and upstairs. What does that tell you — that tells you that clearly the byproducts of cooking are making their way upstairs in our home. As I mentioned, we have a fairly open floor plan downstairs. So clearly we will get more diffusion of cooking byproducts downstairs. But we also these byproducts wafting upstairs.
And the same was true for even the very closed off kitchen in the house I grew up in in rural Illinois. Again, you could be anywhere in the house and smell what’s for dinner. Why? Because air flows and moves around the house.
So, in reality, this air mixing is helping to dilute the NO2 levels. How is it diluting the NO2 levels?
Because the NO2 is moving from the place of highest concentration (near the stove) throughout the house, and thus, it is being spread across the whole of the cubic feet of air in the house. So, rather than the NO2 being stuck in the 800 cubic feet of airspace in the kitchen (from the earlier example), it’s actually throughout the 16,000 cubic feet of airspace (assuming a 2,000 sq ft home with 8 ft ceilings). That works out to 453 cubic meters. And that works out to:
(68 mg NOx) / (453 m^3) = 0.15 mg/m^3
And that in ppb is:
Concentration (ppb) = 24.45 * 0.15 mg/m3 * (1000 ug/1 mg) * (1/46 g/mol) = 79.7 ppb
That is 5% of the ppb that we calculated assuming the kitchen was a self-enclosed box. And that will dissipate very quickly.
Now, you might be saying, “Wait a second, Doc, 2,000 sq ft is bigger than what I got. I got a 1,000 sq ft loft!” And I’ll say, alright, let’s do the 1,000 sq ft loft. That works out to 226.5 cubic meters.
(68 mg NOx) / (226.5 m^3) = 0.30 mg/m^3
And that in ppb is:
Concentration (ppb) = 24.45 * 0.30 mg/m3 * (1000 ug/1 mg) * (1/46 g/mol) = 159.5 ppb
That is 10% of the ppb that we calculated assuming the kitchen was a self-enclosed box. And again, this amount will dissipate very quickly.
The NAAQS Comparison is Misleading
So, all of that to say, the comparison of the amount of NO2 being produced to the NO2 NAAQS is very misleading. Unless your kitchen is a self-enclosed box, you’re not likely to have an issue where you are violating the NAAQS levels with normal cooking.
Use Your Range Hood
If you are concerned about the amount of NO2 you’re producing — use your range hood. If you don’t have one, get one installed. There are general guidelines for CFMs for range hoods based on the total BTUs of your range.
