The Science Is In: Yom Kippur Air Is Cleaner

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Brian of London is not the messiah, he's a very naughty boy. Since making aliyah in 2009, Brian has blogged at Israellycool. Brian's interests include electric cars, world peace and putting an end to world hunger.

Over at the Times of Israel I’ve just published a completely rewritten post on a theme I’ve explored before here at Israellycool: the complete cessation of road traffic in Israel on Yom Kippur. This starts on Friday night this year.

On Friday night Tel Aviv will resemble a post apocalypse movie: the oil ran out and all we have left are bicycles and roller blades.

Yom Kippur – the Day of Atonement – begins this Friday evening. Many people know Jews don’t eat or drink for 25 hours (sun down to sun down) but few know what actually happens on Yom Kippur in modern, non-religious, Israel.

When I arrived, just over four years ago, Yom Kippur in Tel Aviv took me by complete surprise.

Practically all cars and motor transport will stop. Just not go anywhere. Almost no planes, trains or automobiles will move until Saturday night.

Read the rest (and like it and share it!)

As I’ve mentioned before here at Israellycool, there is a visible, tastable, smellable and hearable difference in the quality of the air on Yom Kippur in Tel Aviv.

While updating that article I found a very recently published scientific paper that completely confirms my own unscientific observations about air quality in Tel Aviv on Yom Kippur. I contacted the author, Ilan Levy, at the Technion in Haifa who replied immediately. I’ll try to pass on the gist of the paper in normal language. Here’s the title and bullet points:

A national day with near zero emissions and its effect on primary and secondary pollutants


  • All anthropogenic emissions are ceased for 25 h during a holyday.
  • NO levels drop by 83–98% at different sites.
  • Ozone increases by 8 ppbv at urban core but decreases by 5 ppbv downwind.
  • The study demonstrates the best case scenario for emissions reduction schemes.
  • Major health benefits are expected from alternatives to fossil fuelled vehicles.

The paper takes data from 15 years of Yom Kippur (Day of Atonement abbreviated to ‘DA’ in the text) and also looks in detail at the year 2001 because similar daily weather patterns in that year means the days before and after Yom Kippur are easier to compare than in many of the other years.

The biggest point to note is that primary pollution: the stuff that comes out of the exhaust pipes of cars or from factory chimneys is almost instantly, massively and measurably reduced. The paper notes that nearly everything in Israel begins stopping from 2pm and by 6pm on the eve of Yom Kippur almost all travel and industry has ceased.

The only things left producing pollution are the large power stations (such as the Reding station in North Tel Aviv) and the very large industrial plants such as oil refineries. All small and medium industry as well as almost all transport stops for 25 hours or more. Having said that, the amount of electricity generated on Yom Kippur is also significantly reduced so even power stations cut their emissions.

I love the following graph of traffic flows on the Ayalon highway in Tel Aviv. The author notes there is some measured traffic but he thinks (correctly I’d say) that the measuring equipment is just recording the large number of cyclists going by!

Traffic flows

Traffic counts on selected road sections along the Ayalon Highway for DA of 2006 (a)


My photo of the Ayalon Highway at 19:30 in the evening. This section is usually bumper to bumper at this time.

My photo of the Ayalon Highway at 19:30 in the evening. This section is usually bumper to bumper at this time.



The paper goes into a lot of detail on a particularly specific effect around primary and secondary pollution. Simply put, secondary pollution is produced when primary pollution (the raw stuff that comes out of the cars and chimneys) reacts with other chemicals in the air and sunlight. The other chemicals can even be things given off by plants and trees!

It turns out that (and this is seen elsewhere in the world on weekends) reducing primary pollution briefly can lead to a small increase in secondary pollution. What they can’t say is what would happen if the reduction in primary pollution carried on much longer than 25 hours.

Even putting that aside, the reductions in Nitrogen Oxide (NO) and Nitrogen dioxide (NO2) are dramatic. The time between the vertical red lines in the graphs below is Yom Kippur. You can easily see how levels of these pollutants, everywhere drop. Other studies have shown that these two pollutants represent a whole range of others and when these drop, all the others drop too.

Composite mean for the days before, during and after DA of 1998e2012 for NO (a), NO2 (b). [CBS in Red is the Central Bus Station in Tel Aviv; UBG in green is an “urban background” reading in north Tel Aviv; DWN in blue is “down wind” of Tel Aviv in the town of Modi’in]

Composite mean for the days before, during and after DA of 1998e2012 for NO (a), NO2 (b). [CBS in Red is the Central Bus Station in Tel Aviv; UBG in green is an “urban background” reading in north Tel Aviv; DWN in blue is “down wind” of Tel Aviv in the town of Modi’in]

I’m leaving out a lot of detail but the big message I’m going to repeat in the author’s own words:

The policy implications of this study are substantial. A change in vehicle fleet to low emission vehicles will have a major impact on both primary and secondary pollution levels over large regions, while possibly increasing secondary pollution levels at the urban core. The health benefits associated with the large regional changes are expected to be far greater than the costs of increase in sec-ondary pollutants (i.e., ozone) at the congested urban core. More- over, the low levels of primary pollutants will benefit not only populations at risk (i.e., young children, elderly and people with existing respiratory or cardiovascular conditions) but also the entire population. This is particularly true for carcinogenic TRAP such as benzene, for which there is no safe lower limit of exposure.

My opinion: if we were to switch to cars like my (still working) Better Place car, and to replace our bus fleet with electric buses, we could make almost every day in Israel like Yom Kippur. For very well understood reasons, this wouldn’t need a vast increase in electricity generation, and as we move away from coal and to gas, this also improves air quality in our cities.

It can happen and, as I’ve proved by driving 25,000km without inconvenience, it really is possible. We would all notice the improvement. I hope that day comes in my lifetime.

Let me finish by wishing all our Jewish readers Gmar Chatima Tova!

It’s Science!

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