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. Broad category including electrode corrosion and exceeding cell activation thresholds.
• Ventricular Fibrillation. Current through the heart can cause issues.
• Electroporation. Above certain voltage 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.
• Specific advice for 'stereostim' devices:
  • Use 'pulse based' waveform generation for higher power efficiency.
  • Use frequency modulation / amplitude modulation for higher power efficiency.
  • If that's not an option, use lower carrier frequency.
• Specific advice for pulse based devices:
  • Reduce the pulse frequency, especially if the pulse frequency is above 100hz.
  • Use higher amplitude, lower pulse width pulses.

Electrochemical

Electrochemical damage refers to damage caused by electrode corrosion or chemical instability of body material in the presence of electricity.

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 to 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, such as chrome-plated steel common in budget electrodes, will corrode and release toxic chemicals 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

Stimulation induced 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 equivalently $k = log_{10}(\frac{Q^2}{A})$ 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 ($15 \text{cm}^2$), $k=1.7$ gives a damage threshold of $27.4\mu C/\text{phase}$. The maximum signal intensity I ever observed for this electrode is $6.6\mu C/\text{phase}$. This suggests the carrier frequency should be at least $446\text{hz}$ to fall in the "safe" region.

For a sheet of omega rubber ($9 \text{cm}^2$) placed on the shaft, $k=1.7$ gives a damage threshold of $21.2\mu C/\text{phase}$. The highest signal intensity I saw for this electrode position is $Q_0 = 7.4\mu C$. This suggests the carrier frequency should be at least $755\text{hz}$ to fall in the "safe" region.

For the sheet of $9 \text{cm}^2$ omega rubber, I've found that signals $k=1.4$ result in 1-2 days of skin irritation, repeated exposure of such signal can cause what looks like thermal burns slightly under the skin, which will take a few days to surface. signals $k=1.10$ result in a few hours of skin irritation. Lower values generally do not result in skin irritation.

Relevant equations

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

minimum safe frequency: $f = \frac{-Q_0}{2 * \tau * (Q_0 - Q_{max})}$.

$Q_0$ 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

Electricity through the heart can affect the heart rythem, known as ventricular fibrillation.

Common advice is to only use electrodes below the waist, to avoid currents near the heart altogether.

For practitioners of nipple stim, common advice is to use one channel per nipple and a device with isolated channels, again to avoid current through the chest cavity.

Although the risk of ventricular fibrillation through normal estim signals appears to be quite overblown, those who put electrodes close to the chest should ensure that the signals fall in safe area not only during normal operation of the device, but also during device hardware failure.

My advice is to avoid electrodes near the chest. If you want to do your own research, this source is helpful:

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.