Exhaustive New Study Looks at Kitchen Exhaust and Household Air Quality

CC BY 2.0. Eggs via Wikipedia

Designing kitchen ventilation that works in really tight houses seems almost impossible.

Kitchen exhaust is a big deal for people interested in energy-efficient homes; the designers work so hard to seal houses up tight and control the ventilation, and then the whole balance gets all messed up by turning on the kitchen exhaust fan. And they are often not sized properly, or are uselessly hanging over islands or too far from the stove. I have called them The most screwed up, badly designed, inappropriately used appliance in your home.

That's why, in Passivhaus designs, where every cubic foot of air is accounted for, designers often use recirculating fans to remove the grease, and depend on the heat recovery ventilator to change the air. But this has been controversial too, as I noted in Should your kitchen have a recirculating or a direct-exhaust hood? I am exhausted just thinking about it.

However, it appears that we now have a definitive answer, in the form of new research from Shelly L. Miller and Ryan E. Militello-Hourigan from the Department of Mechanical Engineering, University of Colorado Boulder. They tested indoor air quality of nine tightly constructed homes, a library and a conventionally built home, seven of which were certified Passivhaus or PHIUS, and published their results as The impacts of cooking and an assessment of indoor air quality in Colorado passive and tightly constructed homes.

The researchers brought their own induction hot plate and frying pan and overcooked a large Grade AA egg for six minutes. (Sorry about the "this is your brain on..."intro) "It should be noted that is a long time to cook a fried egg, and the emissions from this activity are likely higher than a normal fried egg. The goal was to produce substantial emissions that are within the range of normal cooking."

They then tested for fine particulate matter (PM2.5, the killer little particles that get stuck in your lungs) Volatile Organic Compounds (VOCs), Formaldehyde, Carbon Dioxide and Radon. Each home was tested twice, with and without a booster fan running.

Fig. 1. Short-term (20 or 30 min) PM2.5 average concentration after cooking for under each ventilation modes.

© Fig. 1. Short-term (20 or 30 min) PM2.5 average concentration after cooking for under each ventilation modes.

The results are shocking. With respect to the PM2.5:

None of the passive or tight homes tested had range hoods that exhaust directly to the outdoors and instead relied on ventilator exhaust grilles in the kitchen area to remove the pollutants. This strategy, in general, did not prove to be effective. The particulate emissions from the cooking overwhelmed any exhaust removal and spread to the rest of the house. Concentrations remained high for several hours under both normal ventilation and boost conditions.
Number of hours each building exceeded NAAQS 24-hr limit after cooking test.

© Fig. 2 Number of hours each building exceeded NAAQS 24-hr limit after cooking test.

As shown in Fig. 2, one home experienced concentrations above 35 μg/m3 for nearly 10 h after cooking. One would hope that residents would notice the diminished air quality and open a window to increase the ventilation rate, but this may not be realistic or feasible. In the winter months, cold temperatures may prevent residents from doing this. If cooking events like this were a regular occurrence, there could be potential health risks from repeated exposure to high PM2.5.

The results from the leaky conventional house were in fact the best, probably due to the higher infiltration rate of outside air.

Total VOCs were all over the map, but were pushing acceptable concentrations. Formaldehyde was higher than expected, and "all buildings were above the California Office of Environmental Health Hazard Assessment (OEHHA) chronic limit of 9 μg/m3." CO2 levels were also high enough that they might have "a significant impact on cognitive functioning." Seven exceeded recommended limits. Radon exceeded EPA actionable limits in four of the homes.

The researchers conclude:

The passive house concept can be an effective design approach to reduce energy use and improve thermal comfort, but it should not be assumed that this type of building has inherently good indoor air quality. Severe, but not atypical, cooking events drastically reduced the indoor air quality for many hours, and the temporary boost mode that many of the mechanical ventilators feature was ineffective at reducing PM emissions from the cooking activities. ...
In general, the indoor air quality of these homes is acceptable, but increased bedroom ventilation is important to consider, and a directly exhausting range hood is highly recommended for frequent cookers, as these hoods are very effective at reducing fine particulate matter concentrations.

So what conclusions can we draw from this?

  • Install a fan that exhausts to the exterior. Period. As Professor John Straube put it once: "Recirculating range hoods don't solve the IAQ problem. It's like a recirculating toilet. The stuff just goes round and round and never leaves."

I also made some other suggestions after reading earlier research:

  • Just stop putting gas into homes; induction cooktops work really well now.
  • Put ranges against a wall. This is a no-brainer but won’t stop people from putting little hoods over big ranges on islands.
  • Engineer Robert Bean recommends that it be wider than the range, not more than 30 inches from the top, and against a wall. Oh, and the duct runs should be short and straight.
glass kitchen

Alibaba/ glass kitchen/via

I am going to also say it once again: Open kitchens are a bad idea. In China, where they do a lot of flash frying that creates a lot of VOCs and particulates, but want the look of an open kitchen, they often enclose it in glass. We are going to have to start thinking like this if we care about what we are breathing.

stove in sears home

Kitchen stove in Sears Catalog Home built on Sanibel Island, Florida / Lloyd Alter/CC BY 2.0

If you bought a Sears home kit a hundred years ago, they often had a bump-out stuck on the back of the house for the kitchen range. This had a number of benefits; they could avoid heating the entire house (this one is on Sanibel Island in Florida) by opening ventilation on either side. If it caught fire it was less likely to burn the whole house down. Perhaps we have to start thinking about putting our kitchen ranges in a cooking booth or niche.

Or perhaps we should just order in; this is all so difficult.