This week, we’ve enjoyed sunny, mild weather ideal for traveling and last-minute holiday shopping, but not the best for breathing.

On both Sunday and Monday, the air quality in the Washington region rose into the Code Orange range, signifying “unhealthy” pollution levels for sensitive groups, which include individuals with respiratory ailments as well as young children and older adults.

Light winds from the north helped disperse the pollution some Tuesday afternoon, but the forecast suggests that air quality could deteriorate again Wednesday through Friday.

“We are in the middle of a bad air quality event,” tweeted Ryan Stauffer, an expert on air pollution at NASA.

Stauffer noted that fine particle pollution levels at the National Zoo surpassed 35 micrograms per cubic meter of air both Sunday and Monday. Thirty-five micrograms is the threshold level at which fine particle pollution starts becoming unhealthy and is considered out of compliance with air-quality standards (averaged at this level over 24 hours).

Fine particle pollution, sometimes referred to as PM 2.5 (PM stands for particulate matter), is composed of fragments 30 times smaller than a strand of human hair and can be problematic when inhaled, especially for vulnerable groups.

The pollution comes from various sources, according to the Environmental Protection Agency, including construction sites, unpaved roads, fields, smokestacks, fires and power plants.

At pollution concentrations we’ve seen this week, it is fine for most people to spend time outside, but sensitive groups are advised to “reduce prolonged or heavy exertion” and “take more breaks and do less intense activities” if and when pollution levels reach Code Orange (or higher).

Stauffer blamed this week’s compromised air quality on a persistent zone of high pressure lodged over the region, which is helping trap pollutants near the ground. You can see the pollution, given a clear view of the horizon.

“One of the interesting things about this event is that it’s visible,” he said in an interview. “It looks like fog.”

Although bad air quality in the Washington region is often associated with hot, hazy summer days, winter pollution events are not uncommon. Stauffer said they happen on average a couple times each year.

The current event is predicted to persist through at least Friday. High atmospheric pressure, light winds and local wood burning will create “ideal conditions for fine particulate levels to remain in the high moderate range,” wrote the Maryland Department of the Environment.

While the air quality has been compromised in the Washington region, it has been worse in portions of southern Pennsylvania, even reaching Code Red levels in mountain valleys just to the southwest of Pittsburgh, indicating unhealthful air for everyone, not just sensitive groups.