Why is PM2.5 often higher than PM10? Is PM10 still a relevant measure?

Posted on February 2nd 2013
Share: http:faq/2013-02-02/why-is-pm25-often-higher-than-pm10

We got a very good and relevant question this week from Severine P., who asked about pm2.5 concentrations versus pm10 concentrations. What Severine asked in her mail was:

I don't understand why pm2.5 concentrations are often higher than pm10 concentrations.
Aren't PM smaller than 2.5 micro included in PM smaller than 10 micro?
Thank you for your answer
Severine's question is totally correct: PM2.5 are, by definition, particulates smaller than 2.5 micrometers, so indeed, they should also be included as smaller than 10 micrometers.

But the assumption about concentration is not correct: PM2.5 concentration can be smaller than PM10, but yet, the converted AQI value can be higher. For instance, an AQI of 50 for PM2.5 corresponds to 15.5 mg/m3, while it corresponds to 55 mg/m3 for PM10.
So, even if the full PM2.5 15.5 mg/m3 are added to the PM10 concentration, the PM10 AQI still remains much lower than the PM2.5 AQI, For instance, earlier this week, the 东城东四 PM10 concentration was 366, resulting in 216 AQI, while the 东城东四 PM2.5 was 348, resulting in 398 AQI.

In order to double confirm that our understanding is correct, we did contact the world renown Air Quality expert Dr Sarath Guttikunda from urbanemissions.info, who wrote us back:
You are right on these. The differences are due to the differences in the break points for PM2.5 and PM10 and how the epidemiological data is correlated to each of the fractions.


So, the next questions is about the relevance of the PM10, especially in China. It is totally right that most of the time (empirically confirmed), PM2.5 is the dominant value in the AQI. So, do we still need the PM10 measurement? Is it still relevant? And which conditions are reflected when PM10 AQI is higher than the PM2.5? We asked the question again to Dr Sarath Guttikunda, who replied:
There are now new studies presenting evidence that PM2.5 is more harmful than PM10. Physically, this makes sense - smaller than particle, more probability that it will go deeper into the lungs and harm us. This is also one of the important reasons for WHO to push for all countries to have standards for PM2.5.

To the question, do we still need to have PM10 measurement, when we are doing PM2.5 - yes we still do. While the fraction of PM2.5 is higher in the PM10 fraction, for the most cities with pollution from transport and other combustion sources, an often neglected non-combustion source is dust re-suspension (from road dust and dust storms), which forms part of the coarser fraction (PM2.5 to PM10). If we suddenly stop measuring PM10, we will be neglecting this source.

Most cities still measure only PM10 - in China and in India. So, another reason we cannot remove it from the equation.
Many thanks, Dr Sarath, for the quick and professional answers.

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    About the Air Quality and Pollution Measurement:

    About the Air Quality Levels

    AQIAir Pollution LevelHealth ImplicationsCautionary Statement (for PM2.5)
    0 - 50GoodAir quality is considered satisfactory, and air pollution poses little or no riskNone
    51 -100ModerateAir quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
    101-150Unhealthy for Sensitive GroupsMembers of sensitive groups may experience health effects. The general public is not likely to be affected.Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
    151-200UnhealthyEveryone may begin to experience health effects; members of sensitive groups may experience more serious health effectsActive children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion
    201-300Very UnhealthyHealth warnings of emergency conditions. The entire population is more likely to be affected.Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.
    300+HazardousHealth alert: everyone may experience more serious health effectsEveryone should avoid all outdoor exertion

    To know more about Air Quality and Pollution, check the wikipedia Air Quality topic or the airnow guide to Air Quality and Your Health.

    For very useful health advices of Beijing Doctor Richard Saint Cyr MD, check www.myhealthbeijing.com blog.


    Usage Notice: All the Air Quality data are unvalidated at the time of publication, and due to quality assurance these data may be amended, without notice, at any time. The World Air Quality Index project has exercised all reasonable skill and care in compiling the contents of this information and under no circumstances will the World Air Quality Index project team or its agents be liable in contract, tort or otherwise for any loss, injury or damage arising directly or indirectly from the supply of this data.



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