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In Greece, the daily limit for PM10 is the 50 μg/m3 and the annual limit is 40 μg/m3. I think that WHO have proposed lower limits although these have not proposed adopted.

But, there is no hourly limit for the concentration of PM10. PM10 is a very important pollutant with various effects to human health, so I would like to ask if there are rules in other countries or other proposal about establishing hourly limits.

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  • $\begingroup$ Not only in Greece. These thresholds apply EU-wide since 2005, afaik. $\endgroup$
    – dimfalk
    Mar 24 at 12:36

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There are no hourly standards for particulate matter in the USA. However, sensors and metrics are displayed hourly on websites so that the public can assess their near-real-time exposure and act accordingly. Air is extremely dynamic, so a single hourly value does not necessarily represent typical conditions in the area. A longer-term value (or average) is needed to assess what people are breathing regularly. The main point of a regulatory health-based standard is to be able to compel industrial sources to enact controls or some other limitation on their emissions, but we cannot attribute a single hourly value of particulate matter to a particular facility and then expect them to change their practices because of it. Particulate matter comes from a variety of source categories and is quite common, unlike many toxics pollutants which can only be emitted from very particular types of sources (and therefore be attributed to those sources when they are monitored at high levels). Furthermore, regulatory health standards are usually enforced over a longer time period (e.g. annually) by using the average of the highest hourly (or daily) values over that year. So, one hourly (or daily) value above the standard does not equate to an exceedance of the standard.

In the USA, the only pollutants with an hourly regulatory standard (e.g. in the NAAQS) are Carbon Monoxide, Nitrogen Dioxide, and Sulfur Dioxide. These are gaseous pollutants that can be measured at sub-hourly intervals. However, the benchmark for particulate matter measurements uses a filter. So, measuring particulate data hourly would require a new filter to be set every hour, which is impractical. In fact, many PM2.5 regulatory monitors only measure once every 6th day, so that operators have time to go change filters for all monitors across the network. The health effects of particulate matter will also take a long time to become a problem because it is not hazardous like other toxic pollutants can be. Some pollutants can literally be fatal (or cancer causing) within an hour of exposure while typical particulate matter concentrations need to recur for years of your life before acute health problems would typically occur.

Regarding PM10, it is a rather old metric for particulate matter regulatory standards. In general, fine particulate matter (e.g. PM2.5) is more of a health concern. In fact, ultra-fine particles (e.g. PM1.0) is of most concern since the particles can go deep into the lungs. I don't think the USA plans to update PM10 standards, but PM2.5 standards are currently being revised to a lower level.

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