How to Calculate the Hourly PSI: Transcript

Transcript (PDF)

This video is proudly sponsored by Indonesia. Indonesia. Providing you with 11 months of clean air a year. [*Ahem.*] Indonesia. Providing you with a few months of clean air a year. Have you thanked Indonesia yet?

First of all, how do we calculate the 24h PSI?

Step 1. Measure the amount of pollutants in the air. There are 6 different air pollutants that we care about: sulphur dioxide, PM10, nitrogen dioxide, ozone, carbon monoxide, and PM2.5.

So for example, on Nov 8th, at 10am in the North, the 24h sulphur dioxide reading was 8μg/m³. In other words, over the preceding 24 hours, in the North, for every cubic metre of air, there was an average of 8μg of sulphur dioxide.

Step 2. Convert these measurements onto a common scale that runs from 0 to 500. We call this the Pollutant Standards Index or PSI.

How do we do the conversion?

It’s actually damn blardi easy – as I’ll explain in a moment – but if you read the NEA document, it sounds insanely complicated. They give an ugly equation accompanied by a bunch of total nonsense.

“Each sub-index i, is calculated by using a segmented linear function that relates pollutant concentration, Xᵢ to sub-index value, Iᵢ. A segmented linear function“

Diam lah chee-bye!!

Let me explain.

There are six so-called breakpoints or cutoff points for the PSI scale: 50, 100, 200, 300, 400, and 500.

As you probably know, the NEA designates below 50 as good, 50-100 as moderate, 100-200 as unhealthy, 200-300 as very unhealthy, and above 300 as hazardous. 500 is the maximum PSI possible.

For each air pollutant, the NEA similarly designates six breakpoints or cutoff points. For example, for PM2.5, the cutoff points are 12μg/m³, 55, 150, 250, 350, and 500. These seemingly random numbers are taken from the standards used by the US Environmental Protection Agency.

The conversion of PM2.5 onto the PSI scale is done so that the PM2.5 cutoff points match up precisely with the PSI cutoff points. In other words, a PM2.5 measurement of 12μg/m³ gets converted to a PSI of 50. A PM2.5 measurement of 55μg/m³ gets converted to a PSI of 100. And so on and so forth.

And now that we have these 6 points, let’s just join up any two neighbouring points with straight lines.

This red graph that we get at the end of the day tells us precisely how to convert PM2.5 measurements onto the PSI scale.

For example, if the PM2.5 measurement is precisely 109μg/m³, then the PM2.5 PSI is 156.8.

For each of the other 5 pollutants, the PSI conversion is done using exactly the same method, except that the 6 cutoff points for that pollutant are different. For example, for sulphur dioxide, the cutoff points are 80, 365, 800, 1600, 2100, and 2620.

Again, all we need to do is to match up the pollutant cutoff points with the PSI cutoff points. And then connect the dots.

So in this case, if the SO2 measurement is precisely 109μg/m³, then the SO2 PSI is 79.5.

The NEA reports the original pollutant measurements. And next to these measurements are the converted PSI numbers, reported in parentheses.

So for example, on Nov 8th, at 10am in the North, the 24h sulphur dioxide measurement was 8μg/m³ or 5 on the PSI scale. The 24h PM10 measurement was 37μg/m³, which when converted to the PSI scale is coincidentally also 37. And blah. Blah. Blah. Blah.

So altogether we have 6 pollutant measurements, which we convert into 6 pollutant PSIs.

And now for the easiest step – Step 3! Simply take the worst of these 6 different pollutant PSIs and this is your 24h PSI.

So for example, on Nov 8th, at 10am in the North, the 6 pollutant PSIs were 5, 37, not reported, 3, 12, and 59. Of these 6 pollutant PSIs, the worst or the biggest was the PM2.5 PSI, which was 59. So the 24h PSI for the North was simply 59.

Similarly, in the South, the 6 pollutant PSIs were 24, 42, not reported, 6, 10, and 67. The worst of these 6 pollutant PSIs was 67, which again happened to be the PSI for PM2.5. So the 24h PSI for the South was simply 67.

So to recap. How do we calculate the 24h PSI?

Step 1. Measure the 6 different pollutants.

Step 2. Convert these measurements onto the PSI scale.

Step 3. Take the worst of these 6 pollutant PSIs and this is your 24h PSI.

In theory, the 24h PSI depends on all 6 pollutants. But in practice, it almost always depends on one thing and one thing only: the PM2.5. This is because in practice, the PM2.5 PSI is almost always the worst.

And so although in theory the 24h PSI is the worst of the 6 pollutant PSIs, in practice, the PM2.5 PSI is almost always the worst, so the 24h PSI is almost always the PSI for the PM2.5.

Dun believe? Go and check the 24h PSI right now and you’ll see that I’m correct. The 24h PSIs for the 5 different regions are simply equal to the PSIs for the 24h PM2.5.

Similarly, the 3h PSI depends only on the 3h PM2.5. You can read more about this in the description below.

Because the NEA is idiotic, we don’t have any hourly PSI. But if there were an hourly PSI, it would similarly depend on one thing and one thing only. Namely, the hourly PM2.5.

It turns out that the NEA already publishes the hourly PM2.5. So if we wanted to, we could convert these PM2.5 measurements into the PSI scale to get an hourly PSI. Indeed, as some of you probably already know, there are actually quite a few websites out there that already do precisely this.

But actually hor, limpeh got an even better idea. Since the PSI depends only on the PM2.5, why bother thinking in terms of the PSI at all? Just throw away the PSI. And just think solely in terms of the PM2.5 readings.

You’re probably super used to thinking in terms of the PSI scale, after nearly 2 decades of the haze. But getting used to the PM2.5 scale is really easy. Just remember that below 12 is good and above 55 is unhealthy. And for when things get really shitty, which unfortunately happens all too often, 150 is very unhealthy, and 250 is hazardous.

So, conclusion: Forget about the PSI. Just look at the PM2.5. And just remember: 12 good, 55 unhealthy, 150 very unhealthy, and 250 hazardous. These cutoff points for the PM2.5 correspond precisely to the cutoff points for the PSI.

For a more detailed explanation, see the YouTube description below.

“Wah lau! That Jusuf Kanina make until here so hazy!”

“Cannot be as smoky as down here, right?”

Eh still here ah? Econ Cow is a new YouTube channel on economics. Please like, subscribe, and watch my other videos!

(Ya I know this haze video totally wrong theme.)

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