So the raw number these BAM machines give is concentration, given in micrograms per meter cubed. Then they take that number of micrograms and convert it to an AQI. How does this formula work? Unfortunately it’s not as simple as 1 microgram = 2 AQI points ,or 1 microgram = 10 AQI points!<\/p>\n\n\n\n
The actual answer is much weirder. Here’s what it looks like:<\/p>\n\n\n
Those first 10 micrograms count for 42 AQI points! But around the 100 microgram point, adding 10 micrograms contributes just 5 AQI points. Toward the end, it becomes 1 to 1. Sound confused? Here’s a handy PM2.5 & AQI calculator for you to use<\/a>.<\/p>\n\n\n\n This calculation is used for the US AQI scale. The US AQI scale has become the common scale in many other countries as well<\/a>. However, different countries follow different scales. For instance, China uses its own AQI scale<\/a> and India uses a scale it calls National Air Quality Index<\/a> (NAQI).<\/p>\n\n\n\n Both of these are more “forgiving” scales. For example, A PM2.5 concentration of 45 micrograms would be an AQI of 124 in the US but just 75 in India.<\/p>\n\n\n Even the definition of the number is different. For example, an AQI of 151 is labeled \u201cunhealthy\u201d in the US but \u201cmoderate\u201d in India.<\/p>\n\n\n\n The reason governments came up with the AQI scale was to make reporting easier – PM2.5 concentrations can then fall into easy to understand ‘bands’ like ‘0-50′ and ’50-100’. However, because there are so many different scales AQI numbers actually make things more confusing. Oftentimes apps report AQIs without making it clear what scale they\u2019re using (is it the US AQI, or the India AQI, or the China one?). What’s more, some governments decided not to use AQI at all. Singapore, for example, uses PSI or “Pollutant Standards Index”. Even more confusing!<\/p>\n\n\n\n For these reasons, the more I’ve gotten into the nerdery of air quality (such as analyzing what time of day is PM2.5 the lowest<\/a><\/span>), the more I start to ignore AQI and just pay attention to the direct measure – micrograms. Micrograms don’t have ever-changing conversion formulas, and they don’t depend on your government’s scale. They are the same across the world.<\/p>\n\n\n\n But wait, AQI is great because 100 is roughly \u201cbad,\u201d so it\u2019s easy to understand. If we use micrograms, how do we know what’s good and what’s bad? I use the WHO guidelines<\/a><\/span>:<\/p>\n\n\n\n Annual limit = 5 micrograms<\/p>\n\n\n\n 24-hour limit = 25 micrograms<\/p>\n\n\n\n For a reference point, here’s how Beijing’s average PM2.5 from 2014-2020 stacks up against those limits:<\/p>\n\n\nAQI Readings for Other Countries<\/h3>\n\n\n\n
![\"PM2.5](\"\/wordpress\/wp-content\/uploads\/2019\/04\/aqi-vs-naqi.png\")
<\/figure><\/div>\n\n\nWhy I’ve Stopped Using AQI<\/b><\/h2>\n\n\n\n
How to Use Micrograms<\/b><\/h2>\n\n\n\n