There is a lot of bad reporting going on about Popocatépetl in Mexico lately, so I thought it might be a good idea to set everything nice and straight. Some reports are suggesting Popo hasn't erupted since the year 2000. False, it has been in a continuous state of unrest with occasional ash emissions since 2005. That's upwards of 11 years.
The volcano has been having more frequent and more powerful ash emissions, with multiple dome building/collapse events and small pyroclastic flows for quite some time. The media does love a good panic story, so when Mexico raised the alert level to "yellow", they freaked out.
Yes, this is a volcano that is dangerous. It is capable of large eruptions up to VEI 5 (but not since 3700 BCE ± 300 years according to the Smithsonian GVP), however it is more than likely going to be (if it has a serious explosion) a VEI 2-3 event, most likely 2.
The volcano, while quite massive, simply has no recent evidence of an eruption large enough to damage surrounding cities. The real danger from this volcano is not blasts or explosions, rather the fine ash that it generates that can cause lots of problems. It weighs down roofs, it chokes animals and livestock, and creates a fair amount of misery for the surrounding areas.
It can be expected, depending on the weather, that there would be some ashfall in Mexico city, however Mexico City is not threatened in any serious way by Popo's activity. It's simply too far away at some 44 miles. However the surrounding towns such as Amecameca, Atlautla, Ecatzingo, and Tetela del Volcán would be quite a bit more vulnerable due to their immediate proximity to the volcano. All are small(ish) towns surrounded by farmland.
The Yellow alert raised by Mexico is a precautionary measure meant to prepare residents for immediate evacuation if necessary. It is simply an alert status, not a "MASS EVACUATION ALERT" (in bolded caps) as some yellow-journalists out there are parroting. It is meant to have residents have what they need prepared in case Popo does decide to blow. At this time, nobody is actually being evacuated, as some outlets are suggesting.
The base of the volcano is surrounded by dense(ish) forests which don't show any signs of stress (except for human logging) past about 2 miles from the summit. This probably rules out the danger to towns for pyroclastic flows. Gas and ash probably would be the worst it can do to the surrounding areas. At the most, a 10km exclusion zone would probably do the trick to keep people safe.
That does not mean evacuations won't occur if activity increases and heavy ashfall starts making life miserable for people, but it's not happening right now.
I will update this post if anything changes.