The Sierra el Aguaje is the result of roughly 100 million years of volcanic activity, covering the earth with layers of ash and lava, then squeezing it together, twisting, folding and pushing up this range of low mountains. Deposition was followed by erosion. Flowing water gradually carved deep canyons separating the mountains. Some water soaked into the ground, following and filling the layers of porous ash, between layers of impervious lava.

In times more recent past this region was much wetter than it is now and was blanketed in a tropical deciduous forest. As the climate changed, rainfall diminished and the forest receded southward to be replaced with desert vegetation. Despite the desert climate, a few refuges remain. Deep canyons, shaded most of the day and cut so deep as to expose the groundwater can still be found. Here the floor is choked with a great variety of trees and shrubs and draped in vines. Outside of these canyons, such a forest is to be found hundreds of kilometres to the south.

The Nacapule Canyon, near San Carlos, Sonora, is the most famous of these pockets of tropical deciduous forest. Under the protection of CEDES, the Sonoran Commission of Ecology and Sustainable Development, the canyon is now a tourist attraction. It is easily accessible by automobile, and has clean, well-kept trails. Much of the original vegetation is gone now, but more due to the scouring effects of Hurricane Jimena than from the tramplings of visitors. It still is a stunning sight, with vertical cliffs and strangely eroded rocks. The fig tree, from which the canyon gets its name is still there, though some of it was swept away by the hurricane’s floodwaters.

As you hike up the canyon it splits in two. Both routes are worth following. The path to the right follows a small stream past pools of clear water shaded by palms and rock cliffs, and ends in an impressive cave. You can hike out further, which will take you right out of the canyon to a view of the surrounding hills across a broad valley.

The left (or more straight-ahead) route takes you up a rope ladder to a series of pools. Careful searching will yield a glimpse of a frog found nowhere else. Imagine the improbability of finding a frog in the middle of the desert. This route ends for most people in a vertical (usually dry) waterfall above a pool of dark water. The more nimble among us may want to continue, up and out of the canyon to more views of the surrounding desert.

This is not a big hike, but is definitely worth the time to get there, and the twenty peso entry fee. Be sure to bring water and expect to take out with you everything you bring in, and maybe a bit more.

Today someone posted on Facebook, “Save the Planet, Use organic sunscreen, it pollutes less”

Sounds great! what could be better than to save the planet? Why use those awful “chemicals” that “pollute” everything, when you can use “natural” substances, that would never pollute anything?

It turns out there may be a grain of truth in this. According to NOAA’s National Ocean Service, a primary component of many sunscreens, benzophenone-2, will damage and even kill coral polyps. This same compound may also be harmful to the people who spread it on their skin too.

That said, let us explore the idea of “organic” sunscreen. The word organic has a variety of meanings. In Chemistry, an organic molecule is one which is composed of carbon (C), hydrogen (H) and oxygen (O). The term originates from the fact that these three elements are the primary building blocks of all living organisms. Hence organic. But organic compounds do not have to be of biological origin. Most pesticides, from DDT to Agent Orange are organic compounds, even though they are made in a factory.. Organic compounds of natural (biological) origin include some of the most deadly toxic substances known to mankind.So from the point of view of chemistry, organic does not equal safe or non-polluting.

Another use of the word organic comes from agriculture. It is based on the use of fertilizers of organic origin (such as manure), rather than ones manufactured in a chemical factory (such as ammonium nitrate). The term was expanded to include a ban on pesticides as well (though most are organic chemicals, some are not of organic origin).

Various countries and regions have their own system of certifying food as “organic”. In the US, for a product to be certified organic, all of its components have to be certified organic as well, and they all have to be edible. Sunscreen made entirely of edible components should be reasonably expected to degrade quickly in the environment, making it less polluting. Although I am not sure I want to fall asleep in the tropics, slathered with a thin layer, of salad dressing.

But sunscreen has to be effective: it has to either absorb or reflect ultraviolet (UV) light before it is absorbed by your skin. UV is high-energy radiation; so high in energy that it can break chemical bonds. It is this breaking of bonds that allows UV to damage DNA in the skin, and possibly cause cancer. So any component of sunscreen meant to block UV radiation by either absorbing or reflecting it has to resist being broken down by the UV light itself. Benzophenone -2 absorbs UV radiation without photodegrading, and is colourless. That is why it is found in so many sunscreen products. So what substitutes are available?

There are commercially available sunscreens without Benzophenone-2. Some of them use non-photodegradable compounds similar to BP-2. The rest use oxides of zinc or titanium. Zinc oxide is effective, not photodegradable, and is not absorbed into the body as long as it is not ground down to the nano-particle size. It works by reflecting the UV light, so it is bright white. Titanium oxide is similar but less readily available. Either way, these are not sunscreens that you can slather on and look the same: they whiten the skin.

i had a read through some home-made sunscreens. Essentially they are made up of three main components:

1. Something to reflect sunlight: oxides of Ti or Zn.

2. Something to mix #1 and spread a thin layer over the skin and keep it there. This is usually a mixture of oils, like shea butter and coconut oil.

3. A waterproofing agent, beeswax. This thickens the oil and makes it resist being washed away by sweat or swimming. It helps the sunscreen stay on longer, but not forever, so reapply regularly anyway.

4. Preservatives. If you plan to leave your sunscreen out in the heat, you might need a little sodium benzoate to keep it from spoiling. Otherwise, just make small batches. and keep it in the fridge.

5. Everything else is for scent or colour. 

Zinc oxide is not organic in the sense that it is an organic compound, nor that it is made by plants or animals. It is a mineral, extracted from the soil and processed to purify it. Somehow it gets a pass though, as people can say it is obtained from natural sources. So is the gasoline in your car, but hey, who’s counting? The point is that it is better than the UV-resistant organic chemicals like benzophenone-2 and all its relatives. 

Basically, any substance with “benz” or “phen” in it is going to be resistant to degrading, which means it will persist in the environment long enough to do weird things to living creatures. When will we learn?

Upshot:

Regular sunscreen is probably harmful to the marine environment. If your sunscreen has Benzophenone-2 in it, it is definitely harmful, at least if you are swimming near corals. Any other benzo-s or pheno-s are likely bad too. If you are going to be swimming or snorkelling in warm seas, leave that stuff behind.

You can make your own sunscreen easily enough out of zinc oxide cream, thinned with baby oil. Or follow any of the many recipes available online.  Prepare to be whitened. 

Tip:

Don’t rely on sunscreen more than you need to. Wear a hat. Stay in the shade unless you are tanning or just can’t avoid it. Wear light clothing with long sleeves and pants as much as possible.