Although the Carabineros have officially restricted the use of the problematic “rubber shot”— which numerous studies have now demonstrated are not rubber, nor behave like rubber—they are still being used to respond to grave physical threats to the police. As a result, persons continue to present to medical facilities with injuries from this ammunition. Furthermore, recent events suggest that citizens not apparently involved in protests may become inadvertent targets for the police. As early indications suggest the Carabineros will continue to use this ammunition, it is therefore of great interest to the public to understand as much as we can about the weapons used by the police.
This article is divided into two parts. First, I will discuss the theoretical ranges at which various levels of ocular damage from projectiles can occur. Second, I will discuss testing of safety glasses and provide some recommendations for citizens interested in protecting themselves.
At what ranges will ocular damage occur?
An internal report from the Carabineros, dated to 2012, suggested that rupture of the ocular globe (eye)—the most serious consequence of projectile impacts—was a possible risk of rubber pellet impacts at up to 30 meters shooting distance. The study had various problems, not the least of which was the highly subjective extrapolation of impacts on a 9 mm plywood board to effects on distinct parts of the human body. Nonetheless, the report did establish that these projectiles presented significant hazard and laid down guidelines for their use.
Empirical studies have examined the ballistics of various projectiles impacting human eyes in controlled settings to evaluate the energy necessary to cause various degrees of ocular damage (Duma et al., 2005; Kennedy et al., 2006). These studies have used normalized energy—kinetic energy over projectile effective area—to make comparisons between projectiles with different ballistics, and estimate a threshold energy necessary to provoke ocular damage. In a previous post, I calculated probable velocity curves for the “rubber” pellets based on technical spec sheets provided by the Carabineros and the manufacturer of the pellets, as well as an independent analysis of projectile mass by the University of Chile. These data can be used to calculate normalized energy of the “rubber” pellets to estimate the ranges at which these pellets may provoke different kinds of ocular damage.
The results are presented below.
Normalized energy over distance for various projectiles. Curves represent normalized energy over distance for hypothetical pure rubber pellets (blue dashed line), pellets that correspond to those collected and measured by the University of Chile (red solid line), and pellets corresponding to the technical spec sheet provided by the Carabineros (green dotted line). Shaded fields are given that correspond to ranges at which a 50% risk of hyphemia (hemorrhage in the eye chamber) and lens dislocation exists, as well as ranges at which there exists a 50% risk of globe rupture.
• Corneal abrasion : 1,503 kg s-2
• Lens dislocation : 19,194 kg s-2
• Hyphema : 20,188 kg s-2
• Retinal damage : 30,351 kg s-2
• Globe rupture : 23,771 kg s-2 (Duma et al., 2005) or 35,519 J m-2 (Kennedy et al., 2006)
These results suggest significant eye injury is highly likely out to at least 25-30 meters distance, and further injury is possible beyond this range. Corneal abrasion is not pictured here but the energy required to provoke this type of trauma is low; I imagine that the more numerous cases of ocular trauma resulting from rubber shot have resulted from shots at this distance, where individual pellet dispersion would be impossible to control.
Kennedy et al. (2006) reported globe rupture at a minimum normalized energy value of ~29,000 J m -2 , which corresponds roughly to 25-30 meters distance depending on the choice of projectile. Although the 2012 report underestimated the risk of skin penetration, it did a pretty good job of identifying the risk of possible ocular rupture out to this range. So kudos to Dra. Viviana Bustos for accurately identifying this crucial factor.
Obviously, take these results with a grain of salt. So far we have no empirical tests on the actual effects of the “rubber” shot on human subjects ( stay tuned ), so until we have that data these results remain fairly theoretical. The implicit assumption is that projectiles are perfectly rigid and impact the eye perpendicularly; in the real world, it is less likely that either assumption is totally correct all of the time, and therefore these ranges likely overestimate slightly the effective ranges necessary to provoke various degrees of eye damage.
In summary, at distances less than 30 meters, these “less-than-lethal” pellets can blind or cause permanent ocular damage. I would advise that anyone who may come into contact with the Carabineros in a situation where these weapons may be deployed at this range take precautions to protect their eyesight.
Which safety glasses should I use?
First and foremost, almost any eye protection is better than no eye protection. Although I will demonstrate below that common “safety glasses” do not necessarily offer adequate protection, they can still protect from chemical agents and glancing impacts. So if you only have standard safety glasses, wear them anyway, even if they do not provide perfect protection. The exception to this rule are glass lenses, which can actually worsen ocular damage from impacts.
Various ratings systems are used worldwide to evaluate eye protection. Although they follow different standards and tests, most eyewear will be rated to more than one standard; you should be able to find the ratings standards on the commercial packaging. A very common standard for safety glasses is ANSI/ISEA Z87.1, last revised in 2015, that establishes requirements for such things as UV transmittance and impact tolerance. Glasses meeting this standard (example) must survive an impact of a 2.54-cm diameter steel ball dropped from a height of 127 cm, which may not adequately reflect the energy involved in—for example—a shotgun pellet impact.
Slightly better is the ANSI Z87+ (or HVP) rating, which uses more stringent impact tests using heavier and high-velocity projectiles. Glasses and goggles meeting these requirements (example) are often sold as motorcycle or motorsports goggles.
Better still are glasses and goggles designed for warfare. Ratings will depend on the country but most common are eyewear that meet various standards of the US military, namely MIL-PRF-31013 and the more up-to-date MIL-PRF-32432. Glasses and goggles meeting these standards must survive impacts of shrapnel-sized projectiles at ballistic velocities.
Obviously more protection is better, but how do these standards compare numerically? Below I compare normalized energy values of test projectiles, so that we can evaluate how effectively eyewear would resist “rubber” shot described above.
Speaking very broadly, basic safety glasses (ANSI Z87.1) are rated for very low normalized energy values, far below values of projectile normalized energy from “rubber” pellets. Glasses and goggles rated for high-velocity impacts (ANSI Z87+) perform significantly better, and glasses made to military specifications offer the best protection of all.
The main takeaway is if you wish to invest in safety glasses for this particular moment in Chile’s history, go the extra meter and find glasses with at least an ANSI Z87+ rating.
Thanks to N. Campillay for help with research.