Which Hiking Trails Near Los Angles Have the Best Air Quality?
The hiking trails around Los Angeles with the best air quality are those at Santa Anita Canyon in Arcadia and other Angeles National Forest trails nearby to it, Castaic Lake in Castaic, Corral Canyon Loop and Dry Gulch Trail in Malibu, Topanga State Park in Topanga, and the Wildwood Regional Park in Thousand Oaks.
I’ll answer this question in more detail using EPA data, IQAir crowdsourcing data, and some reason based arguments. Let’s get started!
Currently, on the Camp Hike Live California website, I’m covering Topanga State Park, Big Santa Anita Canyon, and Paradise Falls in Wildwood Regional Park, Thousand Oaks. However, these aren’t my only hiking areas, and thus I’ll also include Griffith Park, Castaic Lake, and Corral Canyon, Malibu too in this report, just to give some more diversity to the set.
Part One – EPA Data Based Calculations
I was able to grab data from the EPA’s air quality stations somewhat near Topanga, Paradise Falls, and Griffith, and further was able to get a proxy for Castaic at the Piru Lake station, since it seems to have a similar anthropological footprint on the local geography, and similar distances to Santa Clarita, the San Fernando Valley and Los Angeles – the main sources of pollution for the area. I’ll talk about Santa Anita Canyon and Corral Canyon in a different section, since they are special cases.
Also keep in mind that other hiking spots near these EPA stations will likely have similar air quality characteristics to the sites I’m writing about. So if say, Will Roger’s State Historic Park is what you want air quality info on, then just follow the information about the station I used for Topanga State Park, as these two parks are in the same vicinity.
A lot of the Los Angeles stations don’t report all the different pollutants that the EPA regularly covers under the Clean Air Act at the yearly average basis, on an annual frequency; these long averages, incidentally, being what I’m looking for, regarding my first step in this report. But they do all report the 8 hour ozone concentration, which the area is famous for; its concentration and persistence here being the worst in the United States. So I’ll use ozone as my proxy for overall air quality since it’s likely to on average trump other pollutants in the area, and since when there are multiple Air Quality Index (AQI) scores for a given set of pollutants at a station, the official AQI is the worst of the set. For example, if ozone’s score was 78, PM 2.5’s score was 54, and NO2’s was 30, then the official AQI is 78.
The green dots on this map represent the different stations that at least collect ozone data, and publish an annual mean for the year, each year. I pulled the sites’ arithmetic means for years 2017, 18, and 19 then averaged those to get my first set of numbers. I didn’t want to use the pandemic years since the shut downs likely affected pollution levels, so I just used the closest years abutted against the pandemic. In a few years I will have to redo the calculations for 2022, 23 and 24 to see if light duty vehicle efficiency, migration, or work from home trends do anything to the AQI.
Once I got my averages for each site, the calculations to get the AQI scores was relatively simple. Since all the 8 hour ozone collection averages I looked at fell between the 0-0.54 concentration rage, the below calculations could be modified by just multiplying 50 to the average concentration for each set divided by 0.54. For those interested, I’ll post the EPA methodology:
How to Calculate EPA Air Quality Index Scores Using Pollution Concentrations
Air Quality Index Scores for Los Angeles Based Hiking Spots
Without further ado, the picture of the table below holds my AQI calculations. I also list how far away each EPA station is from the hiking areas in question. And the 3-year average ozone concentrations in ppm:
| Hiking Area | Miles from EPA Station | Average Ozone Concentrations for Years 2017, 18, and 19 in ppm | Calculated AQI |
| Topanga State Park | 7 | 0.0430 | 40 |
| Castaic Lake | 12 (using Piru Station as Proxy) | 0.0471 | 44 |
| Griffith Park | 6 | 0.0409 | 38 |
| Paradise Falls – Wildwood Regional Park | 2 | 0.0447 | 41 |
The first thing you’ll notice is that the 3 year average for these sites is fairly well homogenized and that they’re all in the green:
However, this is a bit contradictory to what we hear about Los Angeles:
But apparently these types of analysis aren’t looking at the average concentration from hundreds of yearly samples, according to their methodology. They are looking at high anomalies. I’ll paste their methodology below:
“Data are used to calculate the eight-hour peak concentration for ozone. The fourth-highest day of the year is identified and then reported as a three-year rolling average to minimize the impact of short-term variations in weather conditions in any given year.”
This is understandable as the arithmetic mean concentrations implies that a significant portion of the time, ozone concentrations are sitting above that number.
Using this methodology, and the same EPA station data provided online, this is what I came up with:
| Hiking Area | Miles from EPA Station | Fourth Worst Day Ozone Concentration Average for Years 2017, 18, and 19 | Calculated AQI |
| Topanga State Park | 7 | 0.066 | 87 |
| Castaic Lake | 12 (Piru Station Proxy) | 0.071 | 101 |
| Griffith Park | 6 | 0.072 | 105 |
| Paradise Falls – Wildwood Regional Park | 2 | 0.069 | 97 |
Notably Topanga’s AQI is significantly below the other hiking spots, and the Piru Station – our proxy for Castaic Lake – is up in the low 100’s. This could be because Topanga is much closer to the ocean breeze, pushing in clean ocean air into the basin, while the Piru Station is caught in a narrow valley between two mountain sets, which traps in the pollution; ozone being heavier than air. If such is true, we may need a different method in evaluating Castaic Lake, which we’ll do in the second part of this article.
And we can start looking at percentiles for the data. Below I’ve constructed a table with the EPA reported yearly percentiles for the stations corresponding to our trails; each number in the table is average 8-hour ozone concentration for the three years right before the pandemic:
| Hiking Area | 50th | 75th | 90th | 95th | 98th | 99th |
| Topanga State Park | 0.044 | 0.049 | 0.055 | 0.058 | 0.063 | 0.068 |
| Castaic Lake (Piru Station) | 0.046 | 0.054 | 0.061 | 0.064 | 0.068 | 0.071 |
| Griffith Park | 0.042 | 0.049 | 0.056 | 0.060 | 0.069 | 0.072 |
| Paradise Falls – Wildwood Regional Park | 0.045 | 0.050 | 0.056 | 0.060 | 0.065 | 0.069 |
What we can see is that around 90 percent of the time, Topanga’s AQI for ozone is at 50 or below, signifying satisfactory air quality with little pollution risk. This same story plays out for both Paradise Falls and Griffith Park. And once again we see the Piru Station (that we’re using for Castaic Lake) looking a tad more grim, as only 75% of the time does it keep an AQI of 50 or less. (I’m still questioning if the Piru Station is truly reflective of Castatic so take this with a grain of salt.) This might be more obvious on the percentile series below:
Even 99% of the time both Topanga and Paradise Falls have their AQI sitting below 100.
But lets start taking a look at more spot numbers using crowdsource data from individual PM 2.5 monitors around Los Angeles to see if at least on a relative basis we can replicate the EPA averages.
Part Two: IQAir Real Time Data of Air Quality for Los Angeles Hiking Areas
Now lets look at data collected over the last month by IQAir personal particulate matter monitors in the Los Angeles area. These monitors are individually owned but are connected to the internet through personal WIFI and report their data on the IQAir website.
If you are interested in participating in this crowdsource project or in your home’s AQI, based on the particulate matter data these monitors collect, you can buy them on Amazon (be sure to get the outside version, and not the indoor version): AirVisual Outdoor – Outdoor Air Quality Monitor
Before we look at the broader data, let’s get a better understanding of what the Piru area looks like versus the Castaic Lake area. In Part One of this article, since there was no EPA station at Castaic, I was relying on the EPA’s Piru station to infer Castaic ozone numbers. However, the IQAir program has data for both areas, regarding particulate matter (PM) pollution, and though we don’t want to conflate ozone pollution with PM, since the latter is super fine particles made from burning fuel or wood (forest fire), this may give us some more insight on the ability to use Piru as a pollution proxy for Castaic.
The first thing we notice using data from the previous 30 days, as published on IQAir’s site, is that the two areas are fairly well correlated:
Basically when Castaic’s PM is high, so is Piru’s; and when it’s low, so is Piru’s. We’ve got a correlation coefficient of 0.825 which signifies a good amount of correlation. But what is noteworthy on the two data sets is that on average Castaic’s AQI is 5.48 points less than the Piru’s. Using all data for the two sites to gather an average AQI score of 25.875, this 5.48 difference works out to be a significant 21% difference!
This phenomenon may mean our EPA numbers for Castatic Lake could be lower than what I’ve reported in Part One. Indeed taking a head to head comparison of the AQI based on PM 2.5 for my various of covered trails, Castatic Lake comes out on top, based on the last 30 day’s data:
| Hiking Area | IQAir Station | Average AQI for PM 2.5 for April 16 to May 16, 2022 |
| Castaic Lake | Windy Way, Castaic | 23 |
| Santa Anita Canyon | Mount Wilson | 24 |
| Topanga State Park | Colina Drive, Topanga | 27 |
| Paradise Falls, Wildwood Regional Park | Wildwood Elementary, Thousand Oaks | 28 |
| Dry Gultch Trail | Dry Gulch Trail, Malibu | 29 |
| Piru | Piru Elementary | 29 |
| Corral Canyon Trail | Malibu Estates, Malibu | 31 |
| Griffith Park | Central Beldon Dr., Hollywood | 40 |
Big Santa Anita Canyon, as represented by the Mt. Wilson station comes out as having the second best air quality over the last 30 days. And likely the reason is that it’s at a higher altitude than the temperature inversion that holds a lot of the pollution in the Los Angeles basin. The inversion is a phenomenon where cold marine air sits below hot inland air, and the base of this hot air traps in the pollution from the Los Angeles basin. The altitude of the base varies as a function of seasonal temperatures, such that it’s higher in the summer and lower in the winter. But even in the summer the base is below Santa Anita Canyon’s altitude:
| Hiking Area | Elevation Range (feet above sea level) |
| Topanga State Park | 740 – 1,900 |
| Castaic Lake | 1,100 – 2,000 |
| Griffith Park | 390 – 1,600 |
| Paradise Falls – Wildwood Regional Park | 340 – 1,000 |
| Santa Anita Canyon | 2,180 – 4,100 |
| Corral Canyon, Malibu | 50 – 70 |
Indeed the top two best air quality hikes, based on PM 2.5, over the last 30 days, are the ones with the highest elevation ranges. I captured most of my elevation ranges by examining topography maps on topographic-map.com. Below is an example for the Castaic Lake region:
Topanga and Paradise Falls are not too far removed from the cleanest sites, but the big outlier here is Griffith Park, with a AQI of almost double that of Castaic for the last 30 days, on average. This does make sense, as Griffith is at a lower elevation than these cleaner areas, and is sandwiched between super high density residential areas and a major freeway artery for Los Angeles. Griffith also did poorly with my ozone evaluation so I’d say it’s fairly conclusive that it has relatively poorer air quality than some of the other places.
What was a little surprising was that the Malibu areas scored a similar average AQI of the last 30 days to the Piru Elementary site, even though Piru seemed to not do as well regarding ozone pollution against the other sites. It’s likely though that the PM 2.5 matter for the coastal sites is not the same composition as that of Piru’s mountain valley: marine related PM such as sea salts and ocean aerosols would certainly make up a large percentage of the Malibu PM. There is the caveat that ocean related PM can have anthropogenic sources related to shipping and industrial production from land (Zn, Mn, Pb, Cr, and Ni). But this phenomenon doesn’t seem consistent with the regular wind patterns and location of shipping and industry in relation to these hiking areas. See Chemical Characteristics of Marine Fine Aerosols in the journal, Atmospheric Chemistry and Physics for more on ocean related PM. The conclusion here is that it’s likely the Malibu coastal hikes have better air quality than what is found in the Piru area.
List of the Los Angeles Based Hiking Areas With the Best Air Quality
Top Pick: Big Santa Anita Canyon and surrounding Angeles National Forest Hikes of similar altitude (Switzer Falls trails, Mount Wilson elevated trails, and Red Box Picnic Area related trails). This is the top pick because it’s at enough of an altitude that the bulk of the Los Angeles Basin pollution trapped under its temperature inversion cannot reach it, as evidenced by a particulate matter monitor at Mount Wilson (and a topography map).
Second Choice: Castaic Lake, again because of good particulate matter reads and its altitude, The higher up hikes around the main lake likely have the best air quality.
Third Choice: Malibu coastal hikes – Corral Canyon Loop, Dry Gulch Trail, etc.; Topanga State Park related hikes – Eagle Rock, Mulch Trail, Dead Horse, etc.; and Paradise Falls and Wildwood Regional Park hikes. The Malibu coastal hikes have cleaner ocean air being delivered to their areas which scored decently with the particulate matter based AQI monitor data. Topanga too gets a bit of coastal air and, alongside Paradise Falls/Wildwood, keeps decent ozone and particulate matter scores.