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Before the 1970s, almost nobody in the US was talking about mold in their homes. It was a non-issue.

But starting then — and continuing to surge through today — mold has become a huge health concern and topic of discussion in America. There are undoubtedly many factors, simple awareness and better detection being two of them.

But the biggest factor in the rise of American household mold issues, in my estimation, begins with an unlikely moment: when Egypt, Syria, and an array of allied states launched a surprise attack on Israel on October 6th, 1973, during the country’s observance of the Jewish holy day of Yom Kippur.

The Yom Kippur War

The assault ignited a nineteen-day conflict later dubbed the Yom Kippur War.

In the course of the war, the United States engaged in massive resupply efforts to support Israel, flying hundreds of missions bringing in airplanes, tanks, missile systems, ammunition, electronic jamming equipment, and more.

Because of this support, King Faisal of Saudi Arabia (one of the belligerents in the war) declared an embargo against the United States. And ultimately, the entire Organization of Arab Petroleum Exporting Countries implemented a total oil embargo against countries that had supported Israel during the war, including the US.

The embargo caused the 1973 oil crisis.

That crisis (along with its sister, the 1979 post-Iranian revolution crisis) was a watershed moment for many reasons, but only one of them relates to mold: an aggressive shift towards energy-efficiency in America.

Energy Crisis to Energy Efficiency

Once we realized our energy supply was at risk, we moved to address one of the largest consumptive uses of energy in the country: residential heating and cooling. We added thick insulation, vapor barriers, and high-performance windows to every new home in America. The goal was to create a tight thermal envelope to keep conditioned air in and outside air out.

And it worked!

According to Lawrence Berkeley National Laboratory (LBNL), which tracks the airtightness of homes across America via their Residential Diagnostics Database, we have steadily improved airtightness — making homes five to ten times more airtight than before the 1970s.

We measure this in “ACH50,” which is the number of air changes a house will go through per hour when a 50 Pascal pressure differential is applied. Basically: when the house is slightly depressurized and outdoor air is thus encouraged to rush in through every leak in the building envelope, how much air must be extracted per hour in order to maintain the pressure difference.

Obviously every home is different, and there’s all sorts of reasons for variance. But in general, LBNL estimates that old houses go through approximately 15 ACH50; by the 90s, we had this down to 7-10 ACH50; and in the 2010s, this was down to 2-5 ACH50 — or even lower!

Another way I like to look at this is by converting to what’s called “natural infiltration” — meaning, absent any ventilation system or pressurization, how many air changes does a house go through per hour. The ratio between ACH50 and natural air changes is usually between 18-30 for most single-family homes.

Modern Houses Don’t Breathe

So that means that naturally, old houses go through perhaps 0.6 air changes per hour, and modern ones go through perhaps 0.08 — nearly 10x less leakage! Said another way: it takes an old house an hour or two to totally recycle its air, but a new house twelve hours.

Less leakage is great for energy efficiency — again, it stops your perfectly-centrally-temperature-managed interior air from mixing with outdoors.

But in solving one problem, we created another. We forgot that our old, drafty houses did something crucial: they breathed. The constant air exchange meant that moisture from cooking, showering, and even breathing had a way to escape.

Our new, airtight homes became like terrariums. Moisture gets trapped inside. And when you have moisture, you have the perfect breeding ground for mold.

Over time, we’ve increasingly demanded better mechanical ventilation to support air flow and filtration. But for most homes, it’s still nowhere near as good as it was pre-1970s.

The Perfect Mold Recipe

For mold to appear in your home, you need three things: a mold spore, an appropriate level of moisture for that spore, and some organic matter for the mold to eat (like wood).

American homes from the mid-1970s onward created the perfect storm on these three fronts: the airtightness let incident mold spores bounce around until they found a place to grow; it trapped moisture that the mold needed; and our increasing use of wood-sheathing and wood-framed walls created plenty of “edible” landing ground.

This problem is increasingly recognized, and solutions are beginning to appear. Homes since the 2010s have better ventilation. We’re seeing increasing use of building materials that mold can’t use as food, like autoclaved aerated concrete.

What You Can Do

But if you’re sensitive to or afraid of mold, your best bets remain: move to pre-1970s construction (one which hasn’t been renovated to energy-efficiency standards), or buy, rent, or build something new that is specifically engineered to prevent mold.

In between those two often-difficult options, there’s only so much you can do, because the issue is the home itself — even if you get rid of existing mold, it may come back, thanks to the realities of the structure.

Sufficient air purification helps, as does adding ventilation (including through Energy Recovery Ventilators and Heat Recovery Ventilators, or ERVs and HRVs), and possibly sealing ingress points from interior walls into the home (outlet enclosure boxes, door hardware, etc.) — plus deeply cleaning the HVAC system and ensuring it is properly engineered for the home and for the avoidance of mold.

But at the end of the day, when you have an underlying structural issue, the risk will remain.

Who would have expected that a 1970s Middle East invasion would lead to such widespread and seemingly-unrelated impacts. Sometimes a butterfly flaps its wings…

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