I will preface by saying I am not an expert in electrostim safety. I looked over the internet for various resources in the hope it will be useful.

There are various pathways to estim-induced nerve damage:

• Thermal burns. Electricity causes heat. Heat results in 'ordinary' burns.
• Electrochemical. Can cause electrode corrosion, as well as various other damage mechanisms.
• Ventricular Fibrillation. Current through the heart can cause issues.
• Electroporation. Above certain thresholds, the cells fall apart.

Thermal burns

One source states:

The heating potential of an electrical stimulus is largely determined by the rms current, almost independent of frequency. For a small finger touch contact of 25mm^2, a perceptible thermal reaction is reached at 37mA-rms, and a painful thermal reaction is reached at about 45mA-rms over the frequency range 100Hz to 3 MHz (see Chatterjee et al., 1986 and Fig. 7.12). Greater thresholds would apply to larger areas of contact.

In order to protect against thermal burns, UL recommended a limit of 25mA rms (UL, 1981); more recently, this recommended limit has been increased to 70mA rms-a value consistent with that recommended by the IEC for application to electrical equipment for measurement, control, and laboratory use (IEC, 1990a).

Source: Applied Bioelectricity - From Electrical Stimulation to Electropathology

IEC 60601-2-10 recommends 50mA < 400hz, 80mA < 1500hz and 100maA > 1500hz.

Exceeding these limits with pulse waveforms seems extremely unlikely. With continuous waveforms these limits can be reached, but it's not easy. The risk of thermal injuries seems low.

If you suspect thermal injuries, you can try these things:

• Measure the skin temperature during/after stimming to confirm you are dealing with heat-induced damage.
• Use higher quality electrodes, lower electrode-tissue resistance reduces heat.
• Don't stim for too long, especially when trying new electrode configurations.
• Try pulsed waveforms. These easily are 5x more power efficient than continuous.
• If using continuous waveforms, try lower-frequency waveforms. These are more power efficient.
• When using 312/2B, reduce pulse frequency.

Electrochemical

Electrochemical damage can be caused in many ways. The root cause is usually poor materials, unbalanced waveforms, or inefficient waveforms injecting too much charge.

Homemade lube

Avoid E535 (Sodium ferrocyanide), commonly used in table salt. With electricity this can turn into Hydrogen cyanide (HCN). One user on discord reported serious health issues that went away after table salt was replaced with additive-free sea salt.

Electrode corrosion

At the tissue-electrode interface, chemical reactions occur in order to transfer electric charge into the skin. Under normal conditions with good quality materials and balanced waveforms, these byproducts are either harmless or quickly reversed. However, when using unstable materials or unbalanced waveforms the electrode material can corrode and toxic byproducts can enter the skin.

For high-grade stainless steel, the reversible charge storage is approximately $40-50\mu C/\text{cm}^2$. To avoid electrode corrosion, the net charge of the complete pulsetrain must never exceed this threshold.

If the pulsetrain exceeds the reversible charge storage, you will get irreversible reactions, such as electrode corrosion or hydrogen generation.

Low-grade materials might corrode even without any electricity.

Source: Electrical stimulation of excitable tissue: design of efficacious and safe protocols
By Daniel R. Merrill, Marom Bikson, John G.R. Jefferys
Download link

Other chemical tissue damage

One source says:

The mechanisms for stimulation induced tissue damage are not well understood. Two major classes of mechanisms have been proposed. The first is that tissue damage is caused by intrinsic biological processes as excitable tissue is overstimulated. This is called the mass action theory, and proposes that damage occurs from the induced hyperactivity of many neurons firing, or neurons firing for an extended period of time, thus changing the local environment. Proposed mass action mechanisms include depletion of oxygen or glucose, or changes in ionic concentrations both intracellularly and extracellularly, e.g. an increase in extracellular potassium. In the CNS, excessive release of excitatory neurotransmitters such as glutamate may cause excitotoxicity. The second proposed mechanism for tissue damage is the creation of toxic electrochemical reaction products at the electrode surface during cathodic stimulation at a rate greater than that which can be tolerated by the physiological system.

From: Electrical stimulation of excitable tissue: design of efficacious and safe protocols

Multiple sources refer to diagrams such as these:

The governing equation is $log_{10}(Q/A) = k - log_{10}(Q)$ or $k = log_{10}(\frac{Q^2}{A})$ with $Q$ being the charge-per-phase in $\mu C$ and $A$ being the electrode area in $\text{cm}^2$. Value of $k$ should be chosen between $1.7$ and $2.0$, depending on your safety tolerance.

For a glans loop ($15 \text{cm}^2$), $k=1.85$ gives a damage threshold of $32.6\mu C/\text{phase}$. The highest signal intensity I saw for this electrode position is $Q_0 = 13.5\mu C$, this suggests a minimum safe carrier frequency of $935\text{Hz}$.

For a sheet of omega rubber ($9 \text{cm}^2$) placed on the shaft, $k=1.85$ gives a damage threshold of $25.4\mu C/\text{phase}$. The highest signal intensity I saw for this electrode position is $Q_0 = 14.8\mu C$, this suggests a minimum safe carrier frequency of $1996\text{Hz}$.

These calculations suggest normal e-stim frequencies/intensities are not safe. We're not sure if these equations are applicable since all data in literature is based on electrodes an order of magnitude smaller than what we use. I have anecdotal evidence from my own experiments where signals with $k=2$ to $k=2.3$ result in skin irritation lasting about a week. Lower intensity signals such as $k=1.7$ do not. These results suggest the findings in literature are applicable to larger electrode surfaces.

Relevant equations

maximum safe charge per pulse: $Q_{max} = \sqrt{10^{k} * A}$

Charge-per-pulse (sine wave): $\text{current amplitude} * \frac{1}{2 * \text{frequency}} * \frac{2}{\pi}$

maximum safe pulsewidth: $\text{pulsewidth} = \frac{Q_{max} - Q_0}{Q_0} * \tau$

minimum safe frequency: $\frac{1}{2 * \text{pulsewidth}}$ (sine waves).

$Q_0$ is a measure of signal intensity and $\tau$ a property of the nerves. These can be calculated with the equations from nerve activation

Ventricular Fibrillation

My advice is to avoid electrodes across the chest. If you want to know the details read this source:

Applied Bioelectricity - From Electrical Stimulation to Electropathology.

Electroporation

Typically, the electric field strength needed to rupture muscle cells or cause nerve damage must exceed 100V/cm. Also, it has been noted that elevated temperature increases the likelihood of electroporation. Studies in the peripheral nerve of hogs show that no significant alterations in the evoked response occur at current densities up to 167mA/cm2. This current density level results in an electric field strength of 33.2V/cm, which is below the minimum level of 100V/cm cited by Gaylor et aI. (1989).

Source: Applied Bioelectricity - From Electrical Stimulation to Electropathology

Not a concern. Requires much higher voltages than we use.

Damage from electroploration is non-uniform, even when using large electrodes the damage is concentrated in a few ballpoint-tip sized patches of the skin.